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Source file src/time/time.go

Documentation: time

		 1  // Copyright 2009 The Go Authors. All rights reserved.
		 2  // Use of this source code is governed by a BSD-style
		 3  // license that can be found in the LICENSE file.
		 4  
		 5  // Package time provides functionality for measuring and displaying time.
		 6  //
		 7  // The calendrical calculations always assume a Gregorian calendar, with
		 8  // no leap seconds.
		 9  //
		10  // Monotonic Clocks
		11  //
		12  // Operating systems provide both a “wall clock,” which is subject to
		13  // changes for clock synchronization, and a “monotonic clock,” which is
		14  // not. The general rule is that the wall clock is for telling time and
		15  // the monotonic clock is for measuring time. Rather than split the API,
		16  // in this package the Time returned by time.Now contains both a wall
		17  // clock reading and a monotonic clock reading; later time-telling
		18  // operations use the wall clock reading, but later time-measuring
		19  // operations, specifically comparisons and subtractions, use the
		20  // monotonic clock reading.
		21  //
		22  // For example, this code always computes a positive elapsed time of
		23  // approximately 20 milliseconds, even if the wall clock is changed during
		24  // the operation being timed:
		25  //
		26  //	start := time.Now()
		27  //	... operation that takes 20 milliseconds ...
		28  //	t := time.Now()
		29  //	elapsed := t.Sub(start)
		30  //
		31  // Other idioms, such as time.Since(start), time.Until(deadline), and
		32  // time.Now().Before(deadline), are similarly robust against wall clock
		33  // resets.
		34  //
		35  // The rest of this section gives the precise details of how operations
		36  // use monotonic clocks, but understanding those details is not required
		37  // to use this package.
		38  //
		39  // The Time returned by time.Now contains a monotonic clock reading.
		40  // If Time t has a monotonic clock reading, t.Add adds the same duration to
		41  // both the wall clock and monotonic clock readings to compute the result.
		42  // Because t.AddDate(y, m, d), t.Round(d), and t.Truncate(d) are wall time
		43  // computations, they always strip any monotonic clock reading from their results.
		44  // Because t.In, t.Local, and t.UTC are used for their effect on the interpretation
		45  // of the wall time, they also strip any monotonic clock reading from their results.
		46  // The canonical way to strip a monotonic clock reading is to use t = t.Round(0).
		47  //
		48  // If Times t and u both contain monotonic clock readings, the operations
		49  // t.After(u), t.Before(u), t.Equal(u), and t.Sub(u) are carried out
		50  // using the monotonic clock readings alone, ignoring the wall clock
		51  // readings. If either t or u contains no monotonic clock reading, these
		52  // operations fall back to using the wall clock readings.
		53  //
		54  // On some systems the monotonic clock will stop if the computer goes to sleep.
		55  // On such a system, t.Sub(u) may not accurately reflect the actual
		56  // time that passed between t and u.
		57  //
		58  // Because the monotonic clock reading has no meaning outside
		59  // the current process, the serialized forms generated by t.GobEncode,
		60  // t.MarshalBinary, t.MarshalJSON, and t.MarshalText omit the monotonic
		61  // clock reading, and t.Format provides no format for it. Similarly, the
		62  // constructors time.Date, time.Parse, time.ParseInLocation, and time.Unix,
		63  // as well as the unmarshalers t.GobDecode, t.UnmarshalBinary.
		64  // t.UnmarshalJSON, and t.UnmarshalText always create times with
		65  // no monotonic clock reading.
		66  //
		67  // Note that the Go == operator compares not just the time instant but
		68  // also the Location and the monotonic clock reading. See the
		69  // documentation for the Time type for a discussion of equality
		70  // testing for Time values.
		71  //
		72  // For debugging, the result of t.String does include the monotonic
		73  // clock reading if present. If t != u because of different monotonic clock readings,
		74  // that difference will be visible when printing t.String() and u.String().
		75  //
		76  package time
		77  
		78  import (
		79  	"errors"
		80  	_ "unsafe" // for go:linkname
		81  )
		82  
		83  // A Time represents an instant in time with nanosecond precision.
		84  //
		85  // Programs using times should typically store and pass them as values,
		86  // not pointers. That is, time variables and struct fields should be of
		87  // type time.Time, not *time.Time.
		88  //
		89  // A Time value can be used by multiple goroutines simultaneously except
		90  // that the methods GobDecode, UnmarshalBinary, UnmarshalJSON and
		91  // UnmarshalText are not concurrency-safe.
		92  //
		93  // Time instants can be compared using the Before, After, and Equal methods.
		94  // The Sub method subtracts two instants, producing a Duration.
		95  // The Add method adds a Time and a Duration, producing a Time.
		96  //
		97  // The zero value of type Time is January 1, year 1, 00:00:00.000000000 UTC.
		98  // As this time is unlikely to come up in practice, the IsZero method gives
		99  // a simple way of detecting a time that has not been initialized explicitly.
	 100  //
	 101  // Each Time has associated with it a Location, consulted when computing the
	 102  // presentation form of the time, such as in the Format, Hour, and Year methods.
	 103  // The methods Local, UTC, and In return a Time with a specific location.
	 104  // Changing the location in this way changes only the presentation; it does not
	 105  // change the instant in time being denoted and therefore does not affect the
	 106  // computations described in earlier paragraphs.
	 107  //
	 108  // Representations of a Time value saved by the GobEncode, MarshalBinary,
	 109  // MarshalJSON, and MarshalText methods store the Time.Location's offset, but not
	 110  // the location name. They therefore lose information about Daylight Saving Time.
	 111  //
	 112  // In addition to the required “wall clock” reading, a Time may contain an optional
	 113  // reading of the current process's monotonic clock, to provide additional precision
	 114  // for comparison or subtraction.
	 115  // See the “Monotonic Clocks” section in the package documentation for details.
	 116  //
	 117  // Note that the Go == operator compares not just the time instant but also the
	 118  // Location and the monotonic clock reading. Therefore, Time values should not
	 119  // be used as map or database keys without first guaranteeing that the
	 120  // identical Location has been set for all values, which can be achieved
	 121  // through use of the UTC or Local method, and that the monotonic clock reading
	 122  // has been stripped by setting t = t.Round(0). In general, prefer t.Equal(u)
	 123  // to t == u, since t.Equal uses the most accurate comparison available and
	 124  // correctly handles the case when only one of its arguments has a monotonic
	 125  // clock reading.
	 126  //
	 127  type Time struct {
	 128  	// wall and ext encode the wall time seconds, wall time nanoseconds,
	 129  	// and optional monotonic clock reading in nanoseconds.
	 130  	//
	 131  	// From high to low bit position, wall encodes a 1-bit flag (hasMonotonic),
	 132  	// a 33-bit seconds field, and a 30-bit wall time nanoseconds field.
	 133  	// The nanoseconds field is in the range [0, 999999999].
	 134  	// If the hasMonotonic bit is 0, then the 33-bit field must be zero
	 135  	// and the full signed 64-bit wall seconds since Jan 1 year 1 is stored in ext.
	 136  	// If the hasMonotonic bit is 1, then the 33-bit field holds a 33-bit
	 137  	// unsigned wall seconds since Jan 1 year 1885, and ext holds a
	 138  	// signed 64-bit monotonic clock reading, nanoseconds since process start.
	 139  	wall uint64
	 140  	ext	int64
	 141  
	 142  	// loc specifies the Location that should be used to
	 143  	// determine the minute, hour, month, day, and year
	 144  	// that correspond to this Time.
	 145  	// The nil location means UTC.
	 146  	// All UTC times are represented with loc==nil, never loc==&utcLoc.
	 147  	loc *Location
	 148  }
	 149  
	 150  const (
	 151  	hasMonotonic = 1 << 63
	 152  	maxWall			= wallToInternal + (1<<33 - 1) // year 2157
	 153  	minWall			= wallToInternal							 // year 1885
	 154  	nsecMask		 = 1<<30 - 1
	 155  	nsecShift		= 30
	 156  )
	 157  
	 158  // These helpers for manipulating the wall and monotonic clock readings
	 159  // take pointer receivers, even when they don't modify the time,
	 160  // to make them cheaper to call.
	 161  
	 162  // nsec returns the time's nanoseconds.
	 163  func (t *Time) nsec() int32 {
	 164  	return int32(t.wall & nsecMask)
	 165  }
	 166  
	 167  // sec returns the time's seconds since Jan 1 year 1.
	 168  func (t *Time) sec() int64 {
	 169  	if t.wall&hasMonotonic != 0 {
	 170  		return wallToInternal + int64(t.wall<<1>>(nsecShift+1))
	 171  	}
	 172  	return t.ext
	 173  }
	 174  
	 175  // unixSec returns the time's seconds since Jan 1 1970 (Unix time).
	 176  func (t *Time) unixSec() int64 { return t.sec() + internalToUnix }
	 177  
	 178  // addSec adds d seconds to the time.
	 179  func (t *Time) addSec(d int64) {
	 180  	if t.wall&hasMonotonic != 0 {
	 181  		sec := int64(t.wall << 1 >> (nsecShift + 1))
	 182  		dsec := sec + d
	 183  		if 0 <= dsec && dsec <= 1<<33-1 {
	 184  			t.wall = t.wall&nsecMask | uint64(dsec)<<nsecShift | hasMonotonic
	 185  			return
	 186  		}
	 187  		// Wall second now out of range for packed field.
	 188  		// Move to ext.
	 189  		t.stripMono()
	 190  	}
	 191  
	 192  	// Check if the sum of t.ext and d overflows and handle it properly.
	 193  	sum := t.ext + d
	 194  	if (sum > t.ext) == (d > 0) {
	 195  		t.ext = sum
	 196  	} else if d > 0 {
	 197  		t.ext = 1<<63 - 1
	 198  	} else {
	 199  		t.ext = -(1<<63 - 1)
	 200  	}
	 201  }
	 202  
	 203  // setLoc sets the location associated with the time.
	 204  func (t *Time) setLoc(loc *Location) {
	 205  	if loc == &utcLoc {
	 206  		loc = nil
	 207  	}
	 208  	t.stripMono()
	 209  	t.loc = loc
	 210  }
	 211  
	 212  // stripMono strips the monotonic clock reading in t.
	 213  func (t *Time) stripMono() {
	 214  	if t.wall&hasMonotonic != 0 {
	 215  		t.ext = t.sec()
	 216  		t.wall &= nsecMask
	 217  	}
	 218  }
	 219  
	 220  // setMono sets the monotonic clock reading in t.
	 221  // If t cannot hold a monotonic clock reading,
	 222  // because its wall time is too large,
	 223  // setMono is a no-op.
	 224  func (t *Time) setMono(m int64) {
	 225  	if t.wall&hasMonotonic == 0 {
	 226  		sec := t.ext
	 227  		if sec < minWall || maxWall < sec {
	 228  			return
	 229  		}
	 230  		t.wall |= hasMonotonic | uint64(sec-minWall)<<nsecShift
	 231  	}
	 232  	t.ext = m
	 233  }
	 234  
	 235  // mono returns t's monotonic clock reading.
	 236  // It returns 0 for a missing reading.
	 237  // This function is used only for testing,
	 238  // so it's OK that technically 0 is a valid
	 239  // monotonic clock reading as well.
	 240  func (t *Time) mono() int64 {
	 241  	if t.wall&hasMonotonic == 0 {
	 242  		return 0
	 243  	}
	 244  	return t.ext
	 245  }
	 246  
	 247  // After reports whether the time instant t is after u.
	 248  func (t Time) After(u Time) bool {
	 249  	if t.wall&u.wall&hasMonotonic != 0 {
	 250  		return t.ext > u.ext
	 251  	}
	 252  	ts := t.sec()
	 253  	us := u.sec()
	 254  	return ts > us || ts == us && t.nsec() > u.nsec()
	 255  }
	 256  
	 257  // Before reports whether the time instant t is before u.
	 258  func (t Time) Before(u Time) bool {
	 259  	if t.wall&u.wall&hasMonotonic != 0 {
	 260  		return t.ext < u.ext
	 261  	}
	 262  	ts := t.sec()
	 263  	us := u.sec()
	 264  	return ts < us || ts == us && t.nsec() < u.nsec()
	 265  }
	 266  
	 267  // Equal reports whether t and u represent the same time instant.
	 268  // Two times can be equal even if they are in different locations.
	 269  // For example, 6:00 +0200 and 4:00 UTC are Equal.
	 270  // See the documentation on the Time type for the pitfalls of using == with
	 271  // Time values; most code should use Equal instead.
	 272  func (t Time) Equal(u Time) bool {
	 273  	if t.wall&u.wall&hasMonotonic != 0 {
	 274  		return t.ext == u.ext
	 275  	}
	 276  	return t.sec() == u.sec() && t.nsec() == u.nsec()
	 277  }
	 278  
	 279  // A Month specifies a month of the year (January = 1, ...).
	 280  type Month int
	 281  
	 282  const (
	 283  	January Month = 1 + iota
	 284  	February
	 285  	March
	 286  	April
	 287  	May
	 288  	June
	 289  	July
	 290  	August
	 291  	September
	 292  	October
	 293  	November
	 294  	December
	 295  )
	 296  
	 297  // String returns the English name of the month ("January", "February", ...).
	 298  func (m Month) String() string {
	 299  	if January <= m && m <= December {
	 300  		return longMonthNames[m-1]
	 301  	}
	 302  	buf := make([]byte, 20)
	 303  	n := fmtInt(buf, uint64(m))
	 304  	return "%!Month(" + string(buf[n:]) + ")"
	 305  }
	 306  
	 307  // A Weekday specifies a day of the week (Sunday = 0, ...).
	 308  type Weekday int
	 309  
	 310  const (
	 311  	Sunday Weekday = iota
	 312  	Monday
	 313  	Tuesday
	 314  	Wednesday
	 315  	Thursday
	 316  	Friday
	 317  	Saturday
	 318  )
	 319  
	 320  // String returns the English name of the day ("Sunday", "Monday", ...).
	 321  func (d Weekday) String() string {
	 322  	if Sunday <= d && d <= Saturday {
	 323  		return longDayNames[d]
	 324  	}
	 325  	buf := make([]byte, 20)
	 326  	n := fmtInt(buf, uint64(d))
	 327  	return "%!Weekday(" + string(buf[n:]) + ")"
	 328  }
	 329  
	 330  // Computations on time.
	 331  //
	 332  // The zero value for a Time is defined to be
	 333  //	January 1, year 1, 00:00:00.000000000 UTC
	 334  // which (1) looks like a zero, or as close as you can get in a date
	 335  // (1-1-1 00:00:00 UTC), (2) is unlikely enough to arise in practice to
	 336  // be a suitable "not set" sentinel, unlike Jan 1 1970, and (3) has a
	 337  // non-negative year even in time zones west of UTC, unlike 1-1-0
	 338  // 00:00:00 UTC, which would be 12-31-(-1) 19:00:00 in New York.
	 339  //
	 340  // The zero Time value does not force a specific epoch for the time
	 341  // representation. For example, to use the Unix epoch internally, we
	 342  // could define that to distinguish a zero value from Jan 1 1970, that
	 343  // time would be represented by sec=-1, nsec=1e9. However, it does
	 344  // suggest a representation, namely using 1-1-1 00:00:00 UTC as the
	 345  // epoch, and that's what we do.
	 346  //
	 347  // The Add and Sub computations are oblivious to the choice of epoch.
	 348  //
	 349  // The presentation computations - year, month, minute, and so on - all
	 350  // rely heavily on division and modulus by positive constants. For
	 351  // calendrical calculations we want these divisions to round down, even
	 352  // for negative values, so that the remainder is always positive, but
	 353  // Go's division (like most hardware division instructions) rounds to
	 354  // zero. We can still do those computations and then adjust the result
	 355  // for a negative numerator, but it's annoying to write the adjustment
	 356  // over and over. Instead, we can change to a different epoch so long
	 357  // ago that all the times we care about will be positive, and then round
	 358  // to zero and round down coincide. These presentation routines already
	 359  // have to add the zone offset, so adding the translation to the
	 360  // alternate epoch is cheap. For example, having a non-negative time t
	 361  // means that we can write
	 362  //
	 363  //	sec = t % 60
	 364  //
	 365  // instead of
	 366  //
	 367  //	sec = t % 60
	 368  //	if sec < 0 {
	 369  //		sec += 60
	 370  //	}
	 371  //
	 372  // everywhere.
	 373  //
	 374  // The calendar runs on an exact 400 year cycle: a 400-year calendar
	 375  // printed for 1970-2369 will apply as well to 2370-2769. Even the days
	 376  // of the week match up. It simplifies the computations to choose the
	 377  // cycle boundaries so that the exceptional years are always delayed as
	 378  // long as possible. That means choosing a year equal to 1 mod 400, so
	 379  // that the first leap year is the 4th year, the first missed leap year
	 380  // is the 100th year, and the missed missed leap year is the 400th year.
	 381  // So we'd prefer instead to print a calendar for 2001-2400 and reuse it
	 382  // for 2401-2800.
	 383  //
	 384  // Finally, it's convenient if the delta between the Unix epoch and
	 385  // long-ago epoch is representable by an int64 constant.
	 386  //
	 387  // These three considerations—choose an epoch as early as possible, that
	 388  // uses a year equal to 1 mod 400, and that is no more than 2⁶³ seconds
	 389  // earlier than 1970—bring us to the year -292277022399. We refer to
	 390  // this year as the absolute zero year, and to times measured as a uint64
	 391  // seconds since this year as absolute times.
	 392  //
	 393  // Times measured as an int64 seconds since the year 1—the representation
	 394  // used for Time's sec field—are called internal times.
	 395  //
	 396  // Times measured as an int64 seconds since the year 1970 are called Unix
	 397  // times.
	 398  //
	 399  // It is tempting to just use the year 1 as the absolute epoch, defining
	 400  // that the routines are only valid for years >= 1. However, the
	 401  // routines would then be invalid when displaying the epoch in time zones
	 402  // west of UTC, since it is year 0. It doesn't seem tenable to say that
	 403  // printing the zero time correctly isn't supported in half the time
	 404  // zones. By comparison, it's reasonable to mishandle some times in
	 405  // the year -292277022399.
	 406  //
	 407  // All this is opaque to clients of the API and can be changed if a
	 408  // better implementation presents itself.
	 409  
	 410  const (
	 411  	// The unsigned zero year for internal calculations.
	 412  	// Must be 1 mod 400, and times before it will not compute correctly,
	 413  	// but otherwise can be changed at will.
	 414  	absoluteZeroYear = -292277022399
	 415  
	 416  	// The year of the zero Time.
	 417  	// Assumed by the unixToInternal computation below.
	 418  	internalYear = 1
	 419  
	 420  	// Offsets to convert between internal and absolute or Unix times.
	 421  	absoluteToInternal int64 = (absoluteZeroYear - internalYear) * 365.2425 * secondsPerDay
	 422  	internalToAbsolute			 = -absoluteToInternal
	 423  
	 424  	unixToInternal int64 = (1969*365 + 1969/4 - 1969/100 + 1969/400) * secondsPerDay
	 425  	internalToUnix int64 = -unixToInternal
	 426  
	 427  	wallToInternal int64 = (1884*365 + 1884/4 - 1884/100 + 1884/400) * secondsPerDay
	 428  	internalToWall int64 = -wallToInternal
	 429  )
	 430  
	 431  // IsZero reports whether t represents the zero time instant,
	 432  // January 1, year 1, 00:00:00 UTC.
	 433  func (t Time) IsZero() bool {
	 434  	return t.sec() == 0 && t.nsec() == 0
	 435  }
	 436  
	 437  // abs returns the time t as an absolute time, adjusted by the zone offset.
	 438  // It is called when computing a presentation property like Month or Hour.
	 439  func (t Time) abs() uint64 {
	 440  	l := t.loc
	 441  	// Avoid function calls when possible.
	 442  	if l == nil || l == &localLoc {
	 443  		l = l.get()
	 444  	}
	 445  	sec := t.unixSec()
	 446  	if l != &utcLoc {
	 447  		if l.cacheZone != nil && l.cacheStart <= sec && sec < l.cacheEnd {
	 448  			sec += int64(l.cacheZone.offset)
	 449  		} else {
	 450  			_, offset, _, _, _ := l.lookup(sec)
	 451  			sec += int64(offset)
	 452  		}
	 453  	}
	 454  	return uint64(sec + (unixToInternal + internalToAbsolute))
	 455  }
	 456  
	 457  // locabs is a combination of the Zone and abs methods,
	 458  // extracting both return values from a single zone lookup.
	 459  func (t Time) locabs() (name string, offset int, abs uint64) {
	 460  	l := t.loc
	 461  	if l == nil || l == &localLoc {
	 462  		l = l.get()
	 463  	}
	 464  	// Avoid function call if we hit the local time cache.
	 465  	sec := t.unixSec()
	 466  	if l != &utcLoc {
	 467  		if l.cacheZone != nil && l.cacheStart <= sec && sec < l.cacheEnd {
	 468  			name = l.cacheZone.name
	 469  			offset = l.cacheZone.offset
	 470  		} else {
	 471  			name, offset, _, _, _ = l.lookup(sec)
	 472  		}
	 473  		sec += int64(offset)
	 474  	} else {
	 475  		name = "UTC"
	 476  	}
	 477  	abs = uint64(sec + (unixToInternal + internalToAbsolute))
	 478  	return
	 479  }
	 480  
	 481  // Date returns the year, month, and day in which t occurs.
	 482  func (t Time) Date() (year int, month Month, day int) {
	 483  	year, month, day, _ = t.date(true)
	 484  	return
	 485  }
	 486  
	 487  // Year returns the year in which t occurs.
	 488  func (t Time) Year() int {
	 489  	year, _, _, _ := t.date(false)
	 490  	return year
	 491  }
	 492  
	 493  // Month returns the month of the year specified by t.
	 494  func (t Time) Month() Month {
	 495  	_, month, _, _ := t.date(true)
	 496  	return month
	 497  }
	 498  
	 499  // Day returns the day of the month specified by t.
	 500  func (t Time) Day() int {
	 501  	_, _, day, _ := t.date(true)
	 502  	return day
	 503  }
	 504  
	 505  // Weekday returns the day of the week specified by t.
	 506  func (t Time) Weekday() Weekday {
	 507  	return absWeekday(t.abs())
	 508  }
	 509  
	 510  // absWeekday is like Weekday but operates on an absolute time.
	 511  func absWeekday(abs uint64) Weekday {
	 512  	// January 1 of the absolute year, like January 1 of 2001, was a Monday.
	 513  	sec := (abs + uint64(Monday)*secondsPerDay) % secondsPerWeek
	 514  	return Weekday(int(sec) / secondsPerDay)
	 515  }
	 516  
	 517  // ISOWeek returns the ISO 8601 year and week number in which t occurs.
	 518  // Week ranges from 1 to 53. Jan 01 to Jan 03 of year n might belong to
	 519  // week 52 or 53 of year n-1, and Dec 29 to Dec 31 might belong to week 1
	 520  // of year n+1.
	 521  func (t Time) ISOWeek() (year, week int) {
	 522  	// According to the rule that the first calendar week of a calendar year is
	 523  	// the week including the first Thursday of that year, and that the last one is
	 524  	// the week immediately preceding the first calendar week of the next calendar year.
	 525  	// See https://www.iso.org/obp/ui#iso:std:iso:8601:-1:ed-1:v1:en:term:3.1.1.23 for details.
	 526  
	 527  	// weeks start with Monday
	 528  	// Monday Tuesday Wednesday Thursday Friday Saturday Sunday
	 529  	// 1			2			 3				 4				5			6				7
	 530  	// +3		 +2			+1				0				-1		 -2			 -3
	 531  	// the offset to Thursday
	 532  	abs := t.abs()
	 533  	d := Thursday - absWeekday(abs)
	 534  	// handle Sunday
	 535  	if d == 4 {
	 536  		d = -3
	 537  	}
	 538  	// find the Thursday of the calendar week
	 539  	abs += uint64(d) * secondsPerDay
	 540  	year, _, _, yday := absDate(abs, false)
	 541  	return year, yday/7 + 1
	 542  }
	 543  
	 544  // Clock returns the hour, minute, and second within the day specified by t.
	 545  func (t Time) Clock() (hour, min, sec int) {
	 546  	return absClock(t.abs())
	 547  }
	 548  
	 549  // absClock is like clock but operates on an absolute time.
	 550  func absClock(abs uint64) (hour, min, sec int) {
	 551  	sec = int(abs % secondsPerDay)
	 552  	hour = sec / secondsPerHour
	 553  	sec -= hour * secondsPerHour
	 554  	min = sec / secondsPerMinute
	 555  	sec -= min * secondsPerMinute
	 556  	return
	 557  }
	 558  
	 559  // Hour returns the hour within the day specified by t, in the range [0, 23].
	 560  func (t Time) Hour() int {
	 561  	return int(t.abs()%secondsPerDay) / secondsPerHour
	 562  }
	 563  
	 564  // Minute returns the minute offset within the hour specified by t, in the range [0, 59].
	 565  func (t Time) Minute() int {
	 566  	return int(t.abs()%secondsPerHour) / secondsPerMinute
	 567  }
	 568  
	 569  // Second returns the second offset within the minute specified by t, in the range [0, 59].
	 570  func (t Time) Second() int {
	 571  	return int(t.abs() % secondsPerMinute)
	 572  }
	 573  
	 574  // Nanosecond returns the nanosecond offset within the second specified by t,
	 575  // in the range [0, 999999999].
	 576  func (t Time) Nanosecond() int {
	 577  	return int(t.nsec())
	 578  }
	 579  
	 580  // YearDay returns the day of the year specified by t, in the range [1,365] for non-leap years,
	 581  // and [1,366] in leap years.
	 582  func (t Time) YearDay() int {
	 583  	_, _, _, yday := t.date(false)
	 584  	return yday + 1
	 585  }
	 586  
	 587  // A Duration represents the elapsed time between two instants
	 588  // as an int64 nanosecond count. The representation limits the
	 589  // largest representable duration to approximately 290 years.
	 590  type Duration int64
	 591  
	 592  const (
	 593  	minDuration Duration = -1 << 63
	 594  	maxDuration Duration = 1<<63 - 1
	 595  )
	 596  
	 597  // Common durations. There is no definition for units of Day or larger
	 598  // to avoid confusion across daylight savings time zone transitions.
	 599  //
	 600  // To count the number of units in a Duration, divide:
	 601  //	second := time.Second
	 602  //	fmt.Print(int64(second/time.Millisecond)) // prints 1000
	 603  //
	 604  // To convert an integer number of units to a Duration, multiply:
	 605  //	seconds := 10
	 606  //	fmt.Print(time.Duration(seconds)*time.Second) // prints 10s
	 607  //
	 608  const (
	 609  	Nanosecond	Duration = 1
	 610  	Microsecond					= 1000 * Nanosecond
	 611  	Millisecond					= 1000 * Microsecond
	 612  	Second							 = 1000 * Millisecond
	 613  	Minute							 = 60 * Second
	 614  	Hour								 = 60 * Minute
	 615  )
	 616  
	 617  // String returns a string representing the duration in the form "72h3m0.5s".
	 618  // Leading zero units are omitted. As a special case, durations less than one
	 619  // second format use a smaller unit (milli-, micro-, or nanoseconds) to ensure
	 620  // that the leading digit is non-zero. The zero duration formats as 0s.
	 621  func (d Duration) String() string {
	 622  	// Largest time is 2540400h10m10.000000000s
	 623  	var buf [32]byte
	 624  	w := len(buf)
	 625  
	 626  	u := uint64(d)
	 627  	neg := d < 0
	 628  	if neg {
	 629  		u = -u
	 630  	}
	 631  
	 632  	if u < uint64(Second) {
	 633  		// Special case: if duration is smaller than a second,
	 634  		// use smaller units, like 1.2ms
	 635  		var prec int
	 636  		w--
	 637  		buf[w] = 's'
	 638  		w--
	 639  		switch {
	 640  		case u == 0:
	 641  			return "0s"
	 642  		case u < uint64(Microsecond):
	 643  			// print nanoseconds
	 644  			prec = 0
	 645  			buf[w] = 'n'
	 646  		case u < uint64(Millisecond):
	 647  			// print microseconds
	 648  			prec = 3
	 649  			// U+00B5 'µ' micro sign == 0xC2 0xB5
	 650  			w-- // Need room for two bytes.
	 651  			copy(buf[w:], "µ")
	 652  		default:
	 653  			// print milliseconds
	 654  			prec = 6
	 655  			buf[w] = 'm'
	 656  		}
	 657  		w, u = fmtFrac(buf[:w], u, prec)
	 658  		w = fmtInt(buf[:w], u)
	 659  	} else {
	 660  		w--
	 661  		buf[w] = 's'
	 662  
	 663  		w, u = fmtFrac(buf[:w], u, 9)
	 664  
	 665  		// u is now integer seconds
	 666  		w = fmtInt(buf[:w], u%60)
	 667  		u /= 60
	 668  
	 669  		// u is now integer minutes
	 670  		if u > 0 {
	 671  			w--
	 672  			buf[w] = 'm'
	 673  			w = fmtInt(buf[:w], u%60)
	 674  			u /= 60
	 675  
	 676  			// u is now integer hours
	 677  			// Stop at hours because days can be different lengths.
	 678  			if u > 0 {
	 679  				w--
	 680  				buf[w] = 'h'
	 681  				w = fmtInt(buf[:w], u)
	 682  			}
	 683  		}
	 684  	}
	 685  
	 686  	if neg {
	 687  		w--
	 688  		buf[w] = '-'
	 689  	}
	 690  
	 691  	return string(buf[w:])
	 692  }
	 693  
	 694  // fmtFrac formats the fraction of v/10**prec (e.g., ".12345") into the
	 695  // tail of buf, omitting trailing zeros. It omits the decimal
	 696  // point too when the fraction is 0. It returns the index where the
	 697  // output bytes begin and the value v/10**prec.
	 698  func fmtFrac(buf []byte, v uint64, prec int) (nw int, nv uint64) {
	 699  	// Omit trailing zeros up to and including decimal point.
	 700  	w := len(buf)
	 701  	print := false
	 702  	for i := 0; i < prec; i++ {
	 703  		digit := v % 10
	 704  		print = print || digit != 0
	 705  		if print {
	 706  			w--
	 707  			buf[w] = byte(digit) + '0'
	 708  		}
	 709  		v /= 10
	 710  	}
	 711  	if print {
	 712  		w--
	 713  		buf[w] = '.'
	 714  	}
	 715  	return w, v
	 716  }
	 717  
	 718  // fmtInt formats v into the tail of buf.
	 719  // It returns the index where the output begins.
	 720  func fmtInt(buf []byte, v uint64) int {
	 721  	w := len(buf)
	 722  	if v == 0 {
	 723  		w--
	 724  		buf[w] = '0'
	 725  	} else {
	 726  		for v > 0 {
	 727  			w--
	 728  			buf[w] = byte(v%10) + '0'
	 729  			v /= 10
	 730  		}
	 731  	}
	 732  	return w
	 733  }
	 734  
	 735  // Nanoseconds returns the duration as an integer nanosecond count.
	 736  func (d Duration) Nanoseconds() int64 { return int64(d) }
	 737  
	 738  // Microseconds returns the duration as an integer microsecond count.
	 739  func (d Duration) Microseconds() int64 { return int64(d) / 1e3 }
	 740  
	 741  // Milliseconds returns the duration as an integer millisecond count.
	 742  func (d Duration) Milliseconds() int64 { return int64(d) / 1e6 }
	 743  
	 744  // These methods return float64 because the dominant
	 745  // use case is for printing a floating point number like 1.5s, and
	 746  // a truncation to integer would make them not useful in those cases.
	 747  // Splitting the integer and fraction ourselves guarantees that
	 748  // converting the returned float64 to an integer rounds the same
	 749  // way that a pure integer conversion would have, even in cases
	 750  // where, say, float64(d.Nanoseconds())/1e9 would have rounded
	 751  // differently.
	 752  
	 753  // Seconds returns the duration as a floating point number of seconds.
	 754  func (d Duration) Seconds() float64 {
	 755  	sec := d / Second
	 756  	nsec := d % Second
	 757  	return float64(sec) + float64(nsec)/1e9
	 758  }
	 759  
	 760  // Minutes returns the duration as a floating point number of minutes.
	 761  func (d Duration) Minutes() float64 {
	 762  	min := d / Minute
	 763  	nsec := d % Minute
	 764  	return float64(min) + float64(nsec)/(60*1e9)
	 765  }
	 766  
	 767  // Hours returns the duration as a floating point number of hours.
	 768  func (d Duration) Hours() float64 {
	 769  	hour := d / Hour
	 770  	nsec := d % Hour
	 771  	return float64(hour) + float64(nsec)/(60*60*1e9)
	 772  }
	 773  
	 774  // Truncate returns the result of rounding d toward zero to a multiple of m.
	 775  // If m <= 0, Truncate returns d unchanged.
	 776  func (d Duration) Truncate(m Duration) Duration {
	 777  	if m <= 0 {
	 778  		return d
	 779  	}
	 780  	return d - d%m
	 781  }
	 782  
	 783  // lessThanHalf reports whether x+x < y but avoids overflow,
	 784  // assuming x and y are both positive (Duration is signed).
	 785  func lessThanHalf(x, y Duration) bool {
	 786  	return uint64(x)+uint64(x) < uint64(y)
	 787  }
	 788  
	 789  // Round returns the result of rounding d to the nearest multiple of m.
	 790  // The rounding behavior for halfway values is to round away from zero.
	 791  // If the result exceeds the maximum (or minimum)
	 792  // value that can be stored in a Duration,
	 793  // Round returns the maximum (or minimum) duration.
	 794  // If m <= 0, Round returns d unchanged.
	 795  func (d Duration) Round(m Duration) Duration {
	 796  	if m <= 0 {
	 797  		return d
	 798  	}
	 799  	r := d % m
	 800  	if d < 0 {
	 801  		r = -r
	 802  		if lessThanHalf(r, m) {
	 803  			return d + r
	 804  		}
	 805  		if d1 := d - m + r; d1 < d {
	 806  			return d1
	 807  		}
	 808  		return minDuration // overflow
	 809  	}
	 810  	if lessThanHalf(r, m) {
	 811  		return d - r
	 812  	}
	 813  	if d1 := d + m - r; d1 > d {
	 814  		return d1
	 815  	}
	 816  	return maxDuration // overflow
	 817  }
	 818  
	 819  // Add returns the time t+d.
	 820  func (t Time) Add(d Duration) Time {
	 821  	dsec := int64(d / 1e9)
	 822  	nsec := t.nsec() + int32(d%1e9)
	 823  	if nsec >= 1e9 {
	 824  		dsec++
	 825  		nsec -= 1e9
	 826  	} else if nsec < 0 {
	 827  		dsec--
	 828  		nsec += 1e9
	 829  	}
	 830  	t.wall = t.wall&^nsecMask | uint64(nsec) // update nsec
	 831  	t.addSec(dsec)
	 832  	if t.wall&hasMonotonic != 0 {
	 833  		te := t.ext + int64(d)
	 834  		if d < 0 && te > t.ext || d > 0 && te < t.ext {
	 835  			// Monotonic clock reading now out of range; degrade to wall-only.
	 836  			t.stripMono()
	 837  		} else {
	 838  			t.ext = te
	 839  		}
	 840  	}
	 841  	return t
	 842  }
	 843  
	 844  // Sub returns the duration t-u. If the result exceeds the maximum (or minimum)
	 845  // value that can be stored in a Duration, the maximum (or minimum) duration
	 846  // will be returned.
	 847  // To compute t-d for a duration d, use t.Add(-d).
	 848  func (t Time) Sub(u Time) Duration {
	 849  	if t.wall&u.wall&hasMonotonic != 0 {
	 850  		te := t.ext
	 851  		ue := u.ext
	 852  		d := Duration(te - ue)
	 853  		if d < 0 && te > ue {
	 854  			return maxDuration // t - u is positive out of range
	 855  		}
	 856  		if d > 0 && te < ue {
	 857  			return minDuration // t - u is negative out of range
	 858  		}
	 859  		return d
	 860  	}
	 861  	d := Duration(t.sec()-u.sec())*Second + Duration(t.nsec()-u.nsec())
	 862  	// Check for overflow or underflow.
	 863  	switch {
	 864  	case u.Add(d).Equal(t):
	 865  		return d // d is correct
	 866  	case t.Before(u):
	 867  		return minDuration // t - u is negative out of range
	 868  	default:
	 869  		return maxDuration // t - u is positive out of range
	 870  	}
	 871  }
	 872  
	 873  // Since returns the time elapsed since t.
	 874  // It is shorthand for time.Now().Sub(t).
	 875  func Since(t Time) Duration {
	 876  	var now Time
	 877  	if t.wall&hasMonotonic != 0 {
	 878  		// Common case optimization: if t has monotonic time, then Sub will use only it.
	 879  		now = Time{hasMonotonic, runtimeNano() - startNano, nil}
	 880  	} else {
	 881  		now = Now()
	 882  	}
	 883  	return now.Sub(t)
	 884  }
	 885  
	 886  // Until returns the duration until t.
	 887  // It is shorthand for t.Sub(time.Now()).
	 888  func Until(t Time) Duration {
	 889  	var now Time
	 890  	if t.wall&hasMonotonic != 0 {
	 891  		// Common case optimization: if t has monotonic time, then Sub will use only it.
	 892  		now = Time{hasMonotonic, runtimeNano() - startNano, nil}
	 893  	} else {
	 894  		now = Now()
	 895  	}
	 896  	return t.Sub(now)
	 897  }
	 898  
	 899  // AddDate returns the time corresponding to adding the
	 900  // given number of years, months, and days to t.
	 901  // For example, AddDate(-1, 2, 3) applied to January 1, 2011
	 902  // returns March 4, 2010.
	 903  //
	 904  // AddDate normalizes its result in the same way that Date does,
	 905  // so, for example, adding one month to October 31 yields
	 906  // December 1, the normalized form for November 31.
	 907  func (t Time) AddDate(years int, months int, days int) Time {
	 908  	year, month, day := t.Date()
	 909  	hour, min, sec := t.Clock()
	 910  	return Date(year+years, month+Month(months), day+days, hour, min, sec, int(t.nsec()), t.Location())
	 911  }
	 912  
	 913  const (
	 914  	secondsPerMinute = 60
	 915  	secondsPerHour	 = 60 * secondsPerMinute
	 916  	secondsPerDay		= 24 * secondsPerHour
	 917  	secondsPerWeek	 = 7 * secondsPerDay
	 918  	daysPer400Years	= 365*400 + 97
	 919  	daysPer100Years	= 365*100 + 24
	 920  	daysPer4Years		= 365*4 + 1
	 921  )
	 922  
	 923  // date computes the year, day of year, and when full=true,
	 924  // the month and day in which t occurs.
	 925  func (t Time) date(full bool) (year int, month Month, day int, yday int) {
	 926  	return absDate(t.abs(), full)
	 927  }
	 928  
	 929  // absDate is like date but operates on an absolute time.
	 930  func absDate(abs uint64, full bool) (year int, month Month, day int, yday int) {
	 931  	// Split into time and day.
	 932  	d := abs / secondsPerDay
	 933  
	 934  	// Account for 400 year cycles.
	 935  	n := d / daysPer400Years
	 936  	y := 400 * n
	 937  	d -= daysPer400Years * n
	 938  
	 939  	// Cut off 100-year cycles.
	 940  	// The last cycle has one extra leap year, so on the last day
	 941  	// of that year, day / daysPer100Years will be 4 instead of 3.
	 942  	// Cut it back down to 3 by subtracting n>>2.
	 943  	n = d / daysPer100Years
	 944  	n -= n >> 2
	 945  	y += 100 * n
	 946  	d -= daysPer100Years * n
	 947  
	 948  	// Cut off 4-year cycles.
	 949  	// The last cycle has a missing leap year, which does not
	 950  	// affect the computation.
	 951  	n = d / daysPer4Years
	 952  	y += 4 * n
	 953  	d -= daysPer4Years * n
	 954  
	 955  	// Cut off years within a 4-year cycle.
	 956  	// The last year is a leap year, so on the last day of that year,
	 957  	// day / 365 will be 4 instead of 3. Cut it back down to 3
	 958  	// by subtracting n>>2.
	 959  	n = d / 365
	 960  	n -= n >> 2
	 961  	y += n
	 962  	d -= 365 * n
	 963  
	 964  	year = int(int64(y) + absoluteZeroYear)
	 965  	yday = int(d)
	 966  
	 967  	if !full {
	 968  		return
	 969  	}
	 970  
	 971  	day = yday
	 972  	if isLeap(year) {
	 973  		// Leap year
	 974  		switch {
	 975  		case day > 31+29-1:
	 976  			// After leap day; pretend it wasn't there.
	 977  			day--
	 978  		case day == 31+29-1:
	 979  			// Leap day.
	 980  			month = February
	 981  			day = 29
	 982  			return
	 983  		}
	 984  	}
	 985  
	 986  	// Estimate month on assumption that every month has 31 days.
	 987  	// The estimate may be too low by at most one month, so adjust.
	 988  	month = Month(day / 31)
	 989  	end := int(daysBefore[month+1])
	 990  	var begin int
	 991  	if day >= end {
	 992  		month++
	 993  		begin = end
	 994  	} else {
	 995  		begin = int(daysBefore[month])
	 996  	}
	 997  
	 998  	month++ // because January is 1
	 999  	day = day - begin + 1
	1000  	return
	1001  }
	1002  
	1003  // daysBefore[m] counts the number of days in a non-leap year
	1004  // before month m begins. There is an entry for m=12, counting
	1005  // the number of days before January of next year (365).
	1006  var daysBefore = [...]int32{
	1007  	0,
	1008  	31,
	1009  	31 + 28,
	1010  	31 + 28 + 31,
	1011  	31 + 28 + 31 + 30,
	1012  	31 + 28 + 31 + 30 + 31,
	1013  	31 + 28 + 31 + 30 + 31 + 30,
	1014  	31 + 28 + 31 + 30 + 31 + 30 + 31,
	1015  	31 + 28 + 31 + 30 + 31 + 30 + 31 + 31,
	1016  	31 + 28 + 31 + 30 + 31 + 30 + 31 + 31 + 30,
	1017  	31 + 28 + 31 + 30 + 31 + 30 + 31 + 31 + 30 + 31,
	1018  	31 + 28 + 31 + 30 + 31 + 30 + 31 + 31 + 30 + 31 + 30,
	1019  	31 + 28 + 31 + 30 + 31 + 30 + 31 + 31 + 30 + 31 + 30 + 31,
	1020  }
	1021  
	1022  func daysIn(m Month, year int) int {
	1023  	if m == February && isLeap(year) {
	1024  		return 29
	1025  	}
	1026  	return int(daysBefore[m] - daysBefore[m-1])
	1027  }
	1028  
	1029  // daysSinceEpoch takes a year and returns the number of days from
	1030  // the absolute epoch to the start of that year.
	1031  // This is basically (year - zeroYear) * 365, but accounting for leap days.
	1032  func daysSinceEpoch(year int) uint64 {
	1033  	y := uint64(int64(year) - absoluteZeroYear)
	1034  
	1035  	// Add in days from 400-year cycles.
	1036  	n := y / 400
	1037  	y -= 400 * n
	1038  	d := daysPer400Years * n
	1039  
	1040  	// Add in 100-year cycles.
	1041  	n = y / 100
	1042  	y -= 100 * n
	1043  	d += daysPer100Years * n
	1044  
	1045  	// Add in 4-year cycles.
	1046  	n = y / 4
	1047  	y -= 4 * n
	1048  	d += daysPer4Years * n
	1049  
	1050  	// Add in non-leap years.
	1051  	n = y
	1052  	d += 365 * n
	1053  
	1054  	return d
	1055  }
	1056  
	1057  // Provided by package runtime.
	1058  func now() (sec int64, nsec int32, mono int64)
	1059  
	1060  // runtimeNano returns the current value of the runtime clock in nanoseconds.
	1061  //go:linkname runtimeNano runtime.nanotime
	1062  func runtimeNano() int64
	1063  
	1064  // Monotonic times are reported as offsets from startNano.
	1065  // We initialize startNano to runtimeNano() - 1 so that on systems where
	1066  // monotonic time resolution is fairly low (e.g. Windows 2008
	1067  // which appears to have a default resolution of 15ms),
	1068  // we avoid ever reporting a monotonic time of 0.
	1069  // (Callers may want to use 0 as "time not set".)
	1070  var startNano int64 = runtimeNano() - 1
	1071  
	1072  // Now returns the current local time.
	1073  func Now() Time {
	1074  	sec, nsec, mono := now()
	1075  	mono -= startNano
	1076  	sec += unixToInternal - minWall
	1077  	if uint64(sec)>>33 != 0 {
	1078  		return Time{uint64(nsec), sec + minWall, Local}
	1079  	}
	1080  	return Time{hasMonotonic | uint64(sec)<<nsecShift | uint64(nsec), mono, Local}
	1081  }
	1082  
	1083  func unixTime(sec int64, nsec int32) Time {
	1084  	return Time{uint64(nsec), sec + unixToInternal, Local}
	1085  }
	1086  
	1087  // UTC returns t with the location set to UTC.
	1088  func (t Time) UTC() Time {
	1089  	t.setLoc(&utcLoc)
	1090  	return t
	1091  }
	1092  
	1093  // Local returns t with the location set to local time.
	1094  func (t Time) Local() Time {
	1095  	t.setLoc(Local)
	1096  	return t
	1097  }
	1098  
	1099  // In returns a copy of t representing the same time instant, but
	1100  // with the copy's location information set to loc for display
	1101  // purposes.
	1102  //
	1103  // In panics if loc is nil.
	1104  func (t Time) In(loc *Location) Time {
	1105  	if loc == nil {
	1106  		panic("time: missing Location in call to Time.In")
	1107  	}
	1108  	t.setLoc(loc)
	1109  	return t
	1110  }
	1111  
	1112  // Location returns the time zone information associated with t.
	1113  func (t Time) Location() *Location {
	1114  	l := t.loc
	1115  	if l == nil {
	1116  		l = UTC
	1117  	}
	1118  	return l
	1119  }
	1120  
	1121  // Zone computes the time zone in effect at time t, returning the abbreviated
	1122  // name of the zone (such as "CET") and its offset in seconds east of UTC.
	1123  func (t Time) Zone() (name string, offset int) {
	1124  	name, offset, _, _, _ = t.loc.lookup(t.unixSec())
	1125  	return
	1126  }
	1127  
	1128  // Unix returns t as a Unix time, the number of seconds elapsed
	1129  // since January 1, 1970 UTC. The result does not depend on the
	1130  // location associated with t.
	1131  // Unix-like operating systems often record time as a 32-bit
	1132  // count of seconds, but since the method here returns a 64-bit
	1133  // value it is valid for billions of years into the past or future.
	1134  func (t Time) Unix() int64 {
	1135  	return t.unixSec()
	1136  }
	1137  
	1138  // UnixMilli returns t as a Unix time, the number of milliseconds elapsed since
	1139  // January 1, 1970 UTC. The result is undefined if the Unix time in
	1140  // milliseconds cannot be represented by an int64 (a date more than 292 million
	1141  // years before or after 1970). The result does not depend on the
	1142  // location associated with t.
	1143  func (t Time) UnixMilli() int64 {
	1144  	return t.unixSec()*1e3 + int64(t.nsec())/1e6
	1145  }
	1146  
	1147  // UnixMicro returns t as a Unix time, the number of microseconds elapsed since
	1148  // January 1, 1970 UTC. The result is undefined if the Unix time in
	1149  // microseconds cannot be represented by an int64 (a date before year -290307 or
	1150  // after year 294246). The result does not depend on the location associated
	1151  // with t.
	1152  func (t Time) UnixMicro() int64 {
	1153  	return t.unixSec()*1e6 + int64(t.nsec())/1e3
	1154  }
	1155  
	1156  // UnixNano returns t as a Unix time, the number of nanoseconds elapsed
	1157  // since January 1, 1970 UTC. The result is undefined if the Unix time
	1158  // in nanoseconds cannot be represented by an int64 (a date before the year
	1159  // 1678 or after 2262). Note that this means the result of calling UnixNano
	1160  // on the zero Time is undefined. The result does not depend on the
	1161  // location associated with t.
	1162  func (t Time) UnixNano() int64 {
	1163  	return (t.unixSec())*1e9 + int64(t.nsec())
	1164  }
	1165  
	1166  const timeBinaryVersion byte = 1
	1167  
	1168  // MarshalBinary implements the encoding.BinaryMarshaler interface.
	1169  func (t Time) MarshalBinary() ([]byte, error) {
	1170  	var offsetMin int16 // minutes east of UTC. -1 is UTC.
	1171  
	1172  	if t.Location() == UTC {
	1173  		offsetMin = -1
	1174  	} else {
	1175  		_, offset := t.Zone()
	1176  		if offset%60 != 0 {
	1177  			return nil, errors.New("Time.MarshalBinary: zone offset has fractional minute")
	1178  		}
	1179  		offset /= 60
	1180  		if offset < -32768 || offset == -1 || offset > 32767 {
	1181  			return nil, errors.New("Time.MarshalBinary: unexpected zone offset")
	1182  		}
	1183  		offsetMin = int16(offset)
	1184  	}
	1185  
	1186  	sec := t.sec()
	1187  	nsec := t.nsec()
	1188  	enc := []byte{
	1189  		timeBinaryVersion, // byte 0 : version
	1190  		byte(sec >> 56),	 // bytes 1-8: seconds
	1191  		byte(sec >> 48),
	1192  		byte(sec >> 40),
	1193  		byte(sec >> 32),
	1194  		byte(sec >> 24),
	1195  		byte(sec >> 16),
	1196  		byte(sec >> 8),
	1197  		byte(sec),
	1198  		byte(nsec >> 24), // bytes 9-12: nanoseconds
	1199  		byte(nsec >> 16),
	1200  		byte(nsec >> 8),
	1201  		byte(nsec),
	1202  		byte(offsetMin >> 8), // bytes 13-14: zone offset in minutes
	1203  		byte(offsetMin),
	1204  	}
	1205  
	1206  	return enc, nil
	1207  }
	1208  
	1209  // UnmarshalBinary implements the encoding.BinaryUnmarshaler interface.
	1210  func (t *Time) UnmarshalBinary(data []byte) error {
	1211  	buf := data
	1212  	if len(buf) == 0 {
	1213  		return errors.New("Time.UnmarshalBinary: no data")
	1214  	}
	1215  
	1216  	if buf[0] != timeBinaryVersion {
	1217  		return errors.New("Time.UnmarshalBinary: unsupported version")
	1218  	}
	1219  
	1220  	if len(buf) != /*version*/ 1+ /*sec*/ 8+ /*nsec*/ 4+ /*zone offset*/ 2 {
	1221  		return errors.New("Time.UnmarshalBinary: invalid length")
	1222  	}
	1223  
	1224  	buf = buf[1:]
	1225  	sec := int64(buf[7]) | int64(buf[6])<<8 | int64(buf[5])<<16 | int64(buf[4])<<24 |
	1226  		int64(buf[3])<<32 | int64(buf[2])<<40 | int64(buf[1])<<48 | int64(buf[0])<<56
	1227  
	1228  	buf = buf[8:]
	1229  	nsec := int32(buf[3]) | int32(buf[2])<<8 | int32(buf[1])<<16 | int32(buf[0])<<24
	1230  
	1231  	buf = buf[4:]
	1232  	offset := int(int16(buf[1])|int16(buf[0])<<8) * 60
	1233  
	1234  	*t = Time{}
	1235  	t.wall = uint64(nsec)
	1236  	t.ext = sec
	1237  
	1238  	if offset == -1*60 {
	1239  		t.setLoc(&utcLoc)
	1240  	} else if _, localoff, _, _, _ := Local.lookup(t.unixSec()); offset == localoff {
	1241  		t.setLoc(Local)
	1242  	} else {
	1243  		t.setLoc(FixedZone("", offset))
	1244  	}
	1245  
	1246  	return nil
	1247  }
	1248  
	1249  // TODO(rsc): Remove GobEncoder, GobDecoder, MarshalJSON, UnmarshalJSON in Go 2.
	1250  // The same semantics will be provided by the generic MarshalBinary, MarshalText,
	1251  // UnmarshalBinary, UnmarshalText.
	1252  
	1253  // GobEncode implements the gob.GobEncoder interface.
	1254  func (t Time) GobEncode() ([]byte, error) {
	1255  	return t.MarshalBinary()
	1256  }
	1257  
	1258  // GobDecode implements the gob.GobDecoder interface.
	1259  func (t *Time) GobDecode(data []byte) error {
	1260  	return t.UnmarshalBinary(data)
	1261  }
	1262  
	1263  // MarshalJSON implements the json.Marshaler interface.
	1264  // The time is a quoted string in RFC 3339 format, with sub-second precision added if present.
	1265  func (t Time) MarshalJSON() ([]byte, error) {
	1266  	if y := t.Year(); y < 0 || y >= 10000 {
	1267  		// RFC 3339 is clear that years are 4 digits exactly.
	1268  		// See golang.org/issue/4556#c15 for more discussion.
	1269  		return nil, errors.New("Time.MarshalJSON: year outside of range [0,9999]")
	1270  	}
	1271  
	1272  	b := make([]byte, 0, len(RFC3339Nano)+2)
	1273  	b = append(b, '"')
	1274  	b = t.AppendFormat(b, RFC3339Nano)
	1275  	b = append(b, '"')
	1276  	return b, nil
	1277  }
	1278  
	1279  // UnmarshalJSON implements the json.Unmarshaler interface.
	1280  // The time is expected to be a quoted string in RFC 3339 format.
	1281  func (t *Time) UnmarshalJSON(data []byte) error {
	1282  	// Ignore null, like in the main JSON package.
	1283  	if string(data) == "null" {
	1284  		return nil
	1285  	}
	1286  	// Fractional seconds are handled implicitly by Parse.
	1287  	var err error
	1288  	*t, err = Parse(`"`+RFC3339+`"`, string(data))
	1289  	return err
	1290  }
	1291  
	1292  // MarshalText implements the encoding.TextMarshaler interface.
	1293  // The time is formatted in RFC 3339 format, with sub-second precision added if present.
	1294  func (t Time) MarshalText() ([]byte, error) {
	1295  	if y := t.Year(); y < 0 || y >= 10000 {
	1296  		return nil, errors.New("Time.MarshalText: year outside of range [0,9999]")
	1297  	}
	1298  
	1299  	b := make([]byte, 0, len(RFC3339Nano))
	1300  	return t.AppendFormat(b, RFC3339Nano), nil
	1301  }
	1302  
	1303  // UnmarshalText implements the encoding.TextUnmarshaler interface.
	1304  // The time is expected to be in RFC 3339 format.
	1305  func (t *Time) UnmarshalText(data []byte) error {
	1306  	// Fractional seconds are handled implicitly by Parse.
	1307  	var err error
	1308  	*t, err = Parse(RFC3339, string(data))
	1309  	return err
	1310  }
	1311  
	1312  // Unix returns the local Time corresponding to the given Unix time,
	1313  // sec seconds and nsec nanoseconds since January 1, 1970 UTC.
	1314  // It is valid to pass nsec outside the range [0, 999999999].
	1315  // Not all sec values have a corresponding time value. One such
	1316  // value is 1<<63-1 (the largest int64 value).
	1317  func Unix(sec int64, nsec int64) Time {
	1318  	if nsec < 0 || nsec >= 1e9 {
	1319  		n := nsec / 1e9
	1320  		sec += n
	1321  		nsec -= n * 1e9
	1322  		if nsec < 0 {
	1323  			nsec += 1e9
	1324  			sec--
	1325  		}
	1326  	}
	1327  	return unixTime(sec, int32(nsec))
	1328  }
	1329  
	1330  // UnixMilli returns the local Time corresponding to the given Unix time,
	1331  // msec milliseconds since January 1, 1970 UTC.
	1332  func UnixMilli(msec int64) Time {
	1333  	return Unix(msec/1e3, (msec%1e3)*1e6)
	1334  }
	1335  
	1336  // UnixMicro returns the local Time corresponding to the given Unix time,
	1337  // usec microseconds since January 1, 1970 UTC.
	1338  func UnixMicro(usec int64) Time {
	1339  	return Unix(usec/1e6, (usec%1e6)*1e3)
	1340  }
	1341  
	1342  // IsDST reports whether the time in the configured location is in Daylight Savings Time.
	1343  func (t Time) IsDST() bool {
	1344  	_, _, _, _, isDST := t.loc.lookup(t.Unix())
	1345  	return isDST
	1346  }
	1347  
	1348  func isLeap(year int) bool {
	1349  	return year%4 == 0 && (year%100 != 0 || year%400 == 0)
	1350  }
	1351  
	1352  // norm returns nhi, nlo such that
	1353  //	hi * base + lo == nhi * base + nlo
	1354  //	0 <= nlo < base
	1355  func norm(hi, lo, base int) (nhi, nlo int) {
	1356  	if lo < 0 {
	1357  		n := (-lo-1)/base + 1
	1358  		hi -= n
	1359  		lo += n * base
	1360  	}
	1361  	if lo >= base {
	1362  		n := lo / base
	1363  		hi += n
	1364  		lo -= n * base
	1365  	}
	1366  	return hi, lo
	1367  }
	1368  
	1369  // Date returns the Time corresponding to
	1370  //	yyyy-mm-dd hh:mm:ss + nsec nanoseconds
	1371  // in the appropriate zone for that time in the given location.
	1372  //
	1373  // The month, day, hour, min, sec, and nsec values may be outside
	1374  // their usual ranges and will be normalized during the conversion.
	1375  // For example, October 32 converts to November 1.
	1376  //
	1377  // A daylight savings time transition skips or repeats times.
	1378  // For example, in the United States, March 13, 2011 2:15am never occurred,
	1379  // while November 6, 2011 1:15am occurred twice. In such cases, the
	1380  // choice of time zone, and therefore the time, is not well-defined.
	1381  // Date returns a time that is correct in one of the two zones involved
	1382  // in the transition, but it does not guarantee which.
	1383  //
	1384  // Date panics if loc is nil.
	1385  func Date(year int, month Month, day, hour, min, sec, nsec int, loc *Location) Time {
	1386  	if loc == nil {
	1387  		panic("time: missing Location in call to Date")
	1388  	}
	1389  
	1390  	// Normalize month, overflowing into year.
	1391  	m := int(month) - 1
	1392  	year, m = norm(year, m, 12)
	1393  	month = Month(m) + 1
	1394  
	1395  	// Normalize nsec, sec, min, hour, overflowing into day.
	1396  	sec, nsec = norm(sec, nsec, 1e9)
	1397  	min, sec = norm(min, sec, 60)
	1398  	hour, min = norm(hour, min, 60)
	1399  	day, hour = norm(day, hour, 24)
	1400  
	1401  	// Compute days since the absolute epoch.
	1402  	d := daysSinceEpoch(year)
	1403  
	1404  	// Add in days before this month.
	1405  	d += uint64(daysBefore[month-1])
	1406  	if isLeap(year) && month >= March {
	1407  		d++ // February 29
	1408  	}
	1409  
	1410  	// Add in days before today.
	1411  	d += uint64(day - 1)
	1412  
	1413  	// Add in time elapsed today.
	1414  	abs := d * secondsPerDay
	1415  	abs += uint64(hour*secondsPerHour + min*secondsPerMinute + sec)
	1416  
	1417  	unix := int64(abs) + (absoluteToInternal + internalToUnix)
	1418  
	1419  	// Look for zone offset for expected time, so we can adjust to UTC.
	1420  	// The lookup function expects UTC, so first we pass unix in the
	1421  	// hope that it will not be too close to a zone transition,
	1422  	// and then adjust if it is.
	1423  	_, offset, start, end, _ := loc.lookup(unix)
	1424  	if offset != 0 {
	1425  		utc := unix - int64(offset)
	1426  		// If utc is valid for the time zone we found, then we have the right offset.
	1427  		// If not, we get the correct offset by looking up utc in the location.
	1428  		if utc < start || utc >= end {
	1429  			_, offset, _, _, _ = loc.lookup(utc)
	1430  		}
	1431  		unix -= int64(offset)
	1432  	}
	1433  
	1434  	t := unixTime(unix, int32(nsec))
	1435  	t.setLoc(loc)
	1436  	return t
	1437  }
	1438  
	1439  // Truncate returns the result of rounding t down to a multiple of d (since the zero time).
	1440  // If d <= 0, Truncate returns t stripped of any monotonic clock reading but otherwise unchanged.
	1441  //
	1442  // Truncate operates on the time as an absolute duration since the
	1443  // zero time; it does not operate on the presentation form of the
	1444  // time. Thus, Truncate(Hour) may return a time with a non-zero
	1445  // minute, depending on the time's Location.
	1446  func (t Time) Truncate(d Duration) Time {
	1447  	t.stripMono()
	1448  	if d <= 0 {
	1449  		return t
	1450  	}
	1451  	_, r := div(t, d)
	1452  	return t.Add(-r)
	1453  }
	1454  
	1455  // Round returns the result of rounding t to the nearest multiple of d (since the zero time).
	1456  // The rounding behavior for halfway values is to round up.
	1457  // If d <= 0, Round returns t stripped of any monotonic clock reading but otherwise unchanged.
	1458  //
	1459  // Round operates on the time as an absolute duration since the
	1460  // zero time; it does not operate on the presentation form of the
	1461  // time. Thus, Round(Hour) may return a time with a non-zero
	1462  // minute, depending on the time's Location.
	1463  func (t Time) Round(d Duration) Time {
	1464  	t.stripMono()
	1465  	if d <= 0 {
	1466  		return t
	1467  	}
	1468  	_, r := div(t, d)
	1469  	if lessThanHalf(r, d) {
	1470  		return t.Add(-r)
	1471  	}
	1472  	return t.Add(d - r)
	1473  }
	1474  
	1475  // div divides t by d and returns the quotient parity and remainder.
	1476  // We don't use the quotient parity anymore (round half up instead of round to even)
	1477  // but it's still here in case we change our minds.
	1478  func div(t Time, d Duration) (qmod2 int, r Duration) {
	1479  	neg := false
	1480  	nsec := t.nsec()
	1481  	sec := t.sec()
	1482  	if sec < 0 {
	1483  		// Operate on absolute value.
	1484  		neg = true
	1485  		sec = -sec
	1486  		nsec = -nsec
	1487  		if nsec < 0 {
	1488  			nsec += 1e9
	1489  			sec-- // sec >= 1 before the -- so safe
	1490  		}
	1491  	}
	1492  
	1493  	switch {
	1494  	// Special case: 2d divides 1 second.
	1495  	case d < Second && Second%(d+d) == 0:
	1496  		qmod2 = int(nsec/int32(d)) & 1
	1497  		r = Duration(nsec % int32(d))
	1498  
	1499  	// Special case: d is a multiple of 1 second.
	1500  	case d%Second == 0:
	1501  		d1 := int64(d / Second)
	1502  		qmod2 = int(sec/d1) & 1
	1503  		r = Duration(sec%d1)*Second + Duration(nsec)
	1504  
	1505  	// General case.
	1506  	// This could be faster if more cleverness were applied,
	1507  	// but it's really only here to avoid special case restrictions in the API.
	1508  	// No one will care about these cases.
	1509  	default:
	1510  		// Compute nanoseconds as 128-bit number.
	1511  		sec := uint64(sec)
	1512  		tmp := (sec >> 32) * 1e9
	1513  		u1 := tmp >> 32
	1514  		u0 := tmp << 32
	1515  		tmp = (sec & 0xFFFFFFFF) * 1e9
	1516  		u0x, u0 := u0, u0+tmp
	1517  		if u0 < u0x {
	1518  			u1++
	1519  		}
	1520  		u0x, u0 = u0, u0+uint64(nsec)
	1521  		if u0 < u0x {
	1522  			u1++
	1523  		}
	1524  
	1525  		// Compute remainder by subtracting r<<k for decreasing k.
	1526  		// Quotient parity is whether we subtract on last round.
	1527  		d1 := uint64(d)
	1528  		for d1>>63 != 1 {
	1529  			d1 <<= 1
	1530  		}
	1531  		d0 := uint64(0)
	1532  		for {
	1533  			qmod2 = 0
	1534  			if u1 > d1 || u1 == d1 && u0 >= d0 {
	1535  				// subtract
	1536  				qmod2 = 1
	1537  				u0x, u0 = u0, u0-d0
	1538  				if u0 > u0x {
	1539  					u1--
	1540  				}
	1541  				u1 -= d1
	1542  			}
	1543  			if d1 == 0 && d0 == uint64(d) {
	1544  				break
	1545  			}
	1546  			d0 >>= 1
	1547  			d0 |= (d1 & 1) << 63
	1548  			d1 >>= 1
	1549  		}
	1550  		r = Duration(u0)
	1551  	}
	1552  
	1553  	if neg && r != 0 {
	1554  		// If input was negative and not an exact multiple of d, we computed q, r such that
	1555  		//	q*d + r = -t
	1556  		// But the right answers are given by -(q-1), d-r:
	1557  		//	q*d + r = -t
	1558  		//	-q*d - r = t
	1559  		//	-(q-1)*d + (d - r) = t
	1560  		qmod2 ^= 1
	1561  		r = d - r
	1562  	}
	1563  	return
	1564  }
	1565  

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