Source file src/pkg/time/time.go

     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.
     8	package time
     9	
    10	import "errors"
    11	
    12	// A Time represents an instant in time with nanosecond precision.
    13	//
    14	// Programs using times should typically store and pass them as values,
    15	// not pointers.  That is, time variables and struct fields should be of
    16	// type time.Time, not *time.Time.  A Time value can be used by
    17	// multiple goroutines simultaneously.
    18	//
    19	// Time instants can be compared using the Before, After, and Equal methods.
    20	// The Sub method subtracts two instants, producing a Duration.
    21	// The Add method adds a Time and a Duration, producing a Time.
    22	//
    23	// The zero value of type Time is January 1, year 1, 00:00:00.000000000 UTC.
    24	// As this time is unlikely to come up in practice, the IsZero method gives
    25	// a simple way of detecting a time that has not been initialized explicitly.
    26	//
    27	// Each Time has associated with it a Location, consulted when computing the
    28	// presentation form of the time, such as in the Format, Hour, and Year methods.
    29	// The methods Local, UTC, and In return a Time with a specific location.
    30	// Changing the location in this way changes only the presentation; it does not
    31	// change the instant in time being denoted and therefore does not affect the
    32	// computations described in earlier paragraphs.
    33	//
    34	type Time struct {
    35		// sec gives the number of seconds elapsed since
    36		// January 1, year 1 00:00:00 UTC.
    37		sec int64
    38	
    39		// nsec specifies a non-negative nanosecond
    40		// offset within the second named by Seconds.
    41		// It must be in the range [0, 999999999].
    42		nsec int32
    43	
    44		// loc specifies the Location that should be used to
    45		// determine the minute, hour, month, day, and year
    46		// that correspond to this Time.
    47		// Only the zero Time has a nil Location.
    48		// In that case it is interpreted to mean UTC.
    49		loc *Location
    50	}
    51	
    52	// After reports whether the time instant t is after u.
    53	func (t Time) After(u Time) bool {
    54		return t.sec > u.sec || t.sec == u.sec && t.nsec > u.nsec
    55	}
    56	
    57	// Before reports whether the time instant t is before u.
    58	func (t Time) Before(u Time) bool {
    59		return t.sec < u.sec || t.sec == u.sec && t.nsec < u.nsec
    60	}
    61	
    62	// Equal reports whether t and u represent the same time instant.
    63	// Two times can be equal even if they are in different locations.
    64	// For example, 6:00 +0200 CEST and 4:00 UTC are Equal.
    65	// This comparison is different from using t == u, which also compares
    66	// the locations.
    67	func (t Time) Equal(u Time) bool {
    68		return t.sec == u.sec && t.nsec == u.nsec
    69	}
    70	
    71	// A Month specifies a month of the year (January = 1, ...).
    72	type Month int
    73	
    74	const (
    75		January Month = 1 + iota
    76		February
    77		March
    78		April
    79		May
    80		June
    81		July
    82		August
    83		September
    84		October
    85		November
    86		December
    87	)
    88	
    89	var months = [...]string{
    90		"January",
    91		"February",
    92		"March",
    93		"April",
    94		"May",
    95		"June",
    96		"July",
    97		"August",
    98		"September",
    99		"October",
   100		"November",
   101		"December",
   102	}
   103	
   104	// String returns the English name of the month ("January", "February", ...).
   105	func (m Month) String() string { return months[m-1] }
   106	
   107	// A Weekday specifies a day of the week (Sunday = 0, ...).
   108	type Weekday int
   109	
   110	const (
   111		Sunday Weekday = iota
   112		Monday
   113		Tuesday
   114		Wednesday
   115		Thursday
   116		Friday
   117		Saturday
   118	)
   119	
   120	var days = [...]string{
   121		"Sunday",
   122		"Monday",
   123		"Tuesday",
   124		"Wednesday",
   125		"Thursday",
   126		"Friday",
   127		"Saturday",
   128	}
   129	
   130	// String returns the English name of the day ("Sunday", "Monday", ...).
   131	func (d Weekday) String() string { return days[d] }
   132	
   133	// Computations on time.
   134	//
   135	// The zero value for a Time is defined to be
   136	//	January 1, year 1, 00:00:00.000000000 UTC
   137	// which (1) looks like a zero, or as close as you can get in a date
   138	// (1-1-1 00:00:00 UTC), (2) is unlikely enough to arise in practice to
   139	// be a suitable "not set" sentinel, unlike Jan 1 1970, and (3) has a
   140	// non-negative year even in time zones west of UTC, unlike 1-1-0
   141	// 00:00:00 UTC, which would be 12-31-(-1) 19:00:00 in New York.
   142	//
   143	// The zero Time value does not force a specific epoch for the time
   144	// representation.  For example, to use the Unix epoch internally, we
   145	// could define that to distinguish a zero value from Jan 1 1970, that
   146	// time would be represented by sec=-1, nsec=1e9.  However, it does
   147	// suggest a representation, namely using 1-1-1 00:00:00 UTC as the
   148	// epoch, and that's what we do.
   149	//
   150	// The Add and Sub computations are oblivious to the choice of epoch.
   151	//
   152	// The presentation computations - year, month, minute, and so on - all
   153	// rely heavily on division and modulus by positive constants.  For
   154	// calendrical calculations we want these divisions to round down, even
   155	// for negative values, so that the remainder is always positive, but
   156	// Go's division (like most hardware division instructions) rounds to
   157	// zero.  We can still do those computations and then adjust the result
   158	// for a negative numerator, but it's annoying to write the adjustment
   159	// over and over.  Instead, we can change to a different epoch so long
   160	// ago that all the times we care about will be positive, and then round
   161	// to zero and round down coincide.  These presentation routines already
   162	// have to add the zone offset, so adding the translation to the
   163	// alternate epoch is cheap.  For example, having a non-negative time t
   164	// means that we can write
   165	//
   166	//	sec = t % 60
   167	//
   168	// instead of
   169	//
   170	//	sec = t % 60
   171	//	if sec < 0 {
   172	//		sec += 60
   173	//	}
   174	//
   175	// everywhere.
   176	//
   177	// The calendar runs on an exact 400 year cycle: a 400-year calendar
   178	// printed for 1970-2469 will apply as well to 2470-2869.  Even the days
   179	// of the week match up.  It simplifies the computations to choose the
   180	// cycle boundaries so that the exceptional years are always delayed as
   181	// long as possible.  That means choosing a year equal to 1 mod 400, so
   182	// that the first leap year is the 4th year, the first missed leap year
   183	// is the 100th year, and the missed missed leap year is the 400th year.
   184	// So we'd prefer instead to print a calendar for 2001-2400 and reuse it
   185	// for 2401-2800.
   186	//
   187	// Finally, it's convenient if the delta between the Unix epoch and
   188	// long-ago epoch is representable by an int64 constant.
   189	//
   190	// These three considerations—choose an epoch as early as possible, that
   191	// uses a year equal to 1 mod 400, and that is no more than 2⁶³ seconds
   192	// earlier than 1970—bring us to the year -292277022399.  We refer to
   193	// this year as the absolute zero year, and to times measured as a uint64
   194	// seconds since this year as absolute times.
   195	//
   196	// Times measured as an int64 seconds since the year 1—the representation
   197	// used for Time's sec field—are called internal times.
   198	//
   199	// Times measured as an int64 seconds since the year 1970 are called Unix
   200	// times.
   201	//
   202	// It is tempting to just use the year 1 as the absolute epoch, defining
   203	// that the routines are only valid for years >= 1.  However, the
   204	// routines would then be invalid when displaying the epoch in time zones
   205	// west of UTC, since it is year 0.  It doesn't seem tenable to say that
   206	// printing the zero time correctly isn't supported in half the time
   207	// zones.  By comparison, it's reasonable to mishandle some times in
   208	// the year -292277022399.
   209	//
   210	// All this is opaque to clients of the API and can be changed if a
   211	// better implementation presents itself.
   212	
   213	const (
   214		// The unsigned zero year for internal calculations.
   215		// Must be 1 mod 400, and times before it will not compute correctly,
   216		// but otherwise can be changed at will.
   217		absoluteZeroYear = -292277022399
   218	
   219		// The year of the zero Time.
   220		// Assumed by the unixToInternal computation below.
   221		internalYear = 1
   222	
   223		// The year of the zero Unix time.
   224		unixYear = 1970
   225	
   226		// Offsets to convert between internal and absolute or Unix times.
   227		absoluteToInternal int64 = (absoluteZeroYear - internalYear) * 365.2425 * secondsPerDay
   228		internalToAbsolute       = -absoluteToInternal
   229	
   230		unixToInternal int64 = (1969*365 + 1969/4 - 1969/100 + 1969/400) * secondsPerDay
   231		internalToUnix int64 = -unixToInternal
   232	)
   233	
   234	// IsZero reports whether t represents the zero time instant,
   235	// January 1, year 1, 00:00:00 UTC.
   236	func (t Time) IsZero() bool {
   237		return t.sec == 0 && t.nsec == 0
   238	}
   239	
   240	// abs returns the time t as an absolute time, adjusted by the zone offset.
   241	// It is called when computing a presentation property like Month or Hour.
   242	func (t Time) abs() uint64 {
   243		l := t.loc
   244		if l == nil {
   245			l = &utcLoc
   246		}
   247		// Avoid function call if we hit the local time cache.
   248		sec := t.sec + internalToUnix
   249		if l != &utcLoc {
   250			if l.cacheZone != nil && l.cacheStart <= sec && sec < l.cacheEnd {
   251				sec += int64(l.cacheZone.offset)
   252			} else {
   253				_, offset, _, _, _ := l.lookup(sec)
   254				sec += int64(offset)
   255			}
   256		}
   257		return uint64(sec + (unixToInternal + internalToAbsolute))
   258	}
   259	
   260	// Date returns the year, month, and day in which t occurs.
   261	func (t Time) Date() (year int, month Month, day int) {
   262		year, month, day, _ = t.date(true)
   263		return
   264	}
   265	
   266	// Year returns the year in which t occurs.
   267	func (t Time) Year() int {
   268		year, _, _, _ := t.date(false)
   269		return year
   270	}
   271	
   272	// Month returns the month of the year specified by t.
   273	func (t Time) Month() Month {
   274		_, month, _, _ := t.date(true)
   275		return month
   276	}
   277	
   278	// Day returns the day of the month specified by t.
   279	func (t Time) Day() int {
   280		_, _, day, _ := t.date(true)
   281		return day
   282	}
   283	
   284	// Weekday returns the day of the week specified by t.
   285	func (t Time) Weekday() Weekday {
   286		// January 1 of the absolute year, like January 1 of 2001, was a Monday.
   287		sec := (t.abs() + uint64(Monday)*secondsPerDay) % secondsPerWeek
   288		return Weekday(int(sec) / secondsPerDay)
   289	}
   290	
   291	// ISOWeek returns the ISO 8601 year and week number in which t occurs.
   292	// Week ranges from 1 to 53. Jan 01 to Jan 03 of year n might belong to
   293	// week 52 or 53 of year n-1, and Dec 29 to Dec 31 might belong to week 1
   294	// of year n+1.
   295	func (t Time) ISOWeek() (year, week int) {
   296		year, month, day, yday := t.date(true)
   297		wday := int(t.Weekday()+6) % 7 // weekday but Monday = 0.
   298		const (
   299			Mon int = iota
   300			Tue
   301			Wed
   302			Thu
   303			Fri
   304			Sat
   305			Sun
   306		)
   307	
   308		// Calculate week as number of Mondays in year up to
   309		// and including today, plus 1 because the first week is week 0.
   310		// Putting the + 1 inside the numerator as a + 7 keeps the
   311		// numerator from being negative, which would cause it to
   312		// round incorrectly.
   313		week = (yday - wday + 7) / 7
   314	
   315		// The week number is now correct under the assumption
   316		// that the first Monday of the year is in week 1.
   317		// If Jan 1 is a Tuesday, Wednesday, or Thursday, the first Monday
   318		// is actually in week 2.
   319		jan1wday := (wday - yday + 7*53) % 7
   320		if Tue <= jan1wday && jan1wday <= Thu {
   321			week++
   322		}
   323	
   324		// If the week number is still 0, we're in early January but in
   325		// the last week of last year.
   326		if week == 0 {
   327			year--
   328			week = 52
   329			// A year has 53 weeks when Jan 1 or Dec 31 is a Thursday,
   330			// meaning Jan 1 of the next year is a Friday
   331			// or it was a leap year and Jan 1 of the next year is a Saturday.
   332			if jan1wday == Fri || (jan1wday == Sat && isLeap(year)) {
   333				week++
   334			}
   335		}
   336	
   337		// December 29 to 31 are in week 1 of next year if
   338		// they are after the last Thursday of the year and
   339		// December 31 is a Monday, Tuesday, or Wednesday.
   340		if month == December && day >= 29 && wday < Thu {
   341			if dec31wday := (wday + 31 - day) % 7; Mon <= dec31wday && dec31wday <= Wed {
   342				year++
   343				week = 1
   344			}
   345		}
   346	
   347		return
   348	}
   349	
   350	// Clock returns the hour, minute, and second within the day specified by t.
   351	func (t Time) Clock() (hour, min, sec int) {
   352		sec = int(t.abs() % secondsPerDay)
   353		hour = sec / secondsPerHour
   354		sec -= hour * secondsPerHour
   355		min = sec / secondsPerMinute
   356		sec -= min * secondsPerMinute
   357		return
   358	}
   359	
   360	// Hour returns the hour within the day specified by t, in the range [0, 23].
   361	func (t Time) Hour() int {
   362		return int(t.abs()%secondsPerDay) / secondsPerHour
   363	}
   364	
   365	// Minute returns the minute offset within the hour specified by t, in the range [0, 59].
   366	func (t Time) Minute() int {
   367		return int(t.abs()%secondsPerHour) / secondsPerMinute
   368	}
   369	
   370	// Second returns the second offset within the minute specified by t, in the range [0, 59].
   371	func (t Time) Second() int {
   372		return int(t.abs() % secondsPerMinute)
   373	}
   374	
   375	// Nanosecond returns the nanosecond offset within the second specified by t,
   376	// in the range [0, 999999999].
   377	func (t Time) Nanosecond() int {
   378		return int(t.nsec)
   379	}
   380	
   381	// A Duration represents the elapsed time between two instants
   382	// as an int64 nanosecond count.  The representation limits the
   383	// largest representable duration to approximately 290 years.
   384	type Duration int64
   385	
   386	// Common durations.  There is no definition for units of Day or larger
   387	// to avoid confusion across daylight savings time zone transitions.
   388	//
   389	// To count the number of units in a Duration, divide:
   390	//	second := time.Second
   391	//	fmt.Print(int64(second/time.Millisecond)) // prints 1000
   392	//
   393	// To convert an integer number of units to a Duration, multiply:
   394	//	seconds := 10
   395	//	fmt.Print(time.Duration(seconds)*time.Second) // prints 10s
   396	//
   397	const (
   398		Nanosecond  Duration = 1
   399		Microsecond          = 1000 * Nanosecond
   400		Millisecond          = 1000 * Microsecond
   401		Second               = 1000 * Millisecond
   402		Minute               = 60 * Second
   403		Hour                 = 60 * Minute
   404	)
   405	
   406	// String returns a string representing the duration in the form "72h3m0.5s".
   407	// Leading zero units are omitted.  As a special case, durations less than one
   408	// second format use a smaller unit (milli-, micro-, or nanoseconds) to ensure
   409	// that the leading digit is non-zero.  The zero duration formats as 0,
   410	// with no unit.
   411	func (d Duration) String() string {
   412		// Largest time is 2540400h10m10.000000000s
   413		var buf [32]byte
   414		w := len(buf)
   415	
   416		u := uint64(d)
   417		neg := d < 0
   418		if neg {
   419			u = -u
   420		}
   421	
   422		if u < uint64(Second) {
   423			// Special case: if duration is smaller than a second,
   424			// use smaller units, like 1.2ms
   425			var (
   426				prec int
   427				unit byte
   428			)
   429			switch {
   430			case u == 0:
   431				return "0"
   432			case u < uint64(Microsecond):
   433				// print nanoseconds
   434				prec = 0
   435				unit = 'n'
   436			case u < uint64(Millisecond):
   437				// print microseconds
   438				prec = 3
   439				unit = 'u'
   440			default:
   441				// print milliseconds
   442				prec = 6
   443				unit = 'm'
   444			}
   445			w -= 2
   446			buf[w] = unit
   447			buf[w+1] = 's'
   448			w, u = fmtFrac(buf[:w], u, prec)
   449			w = fmtInt(buf[:w], u)
   450		} else {
   451			w--
   452			buf[w] = 's'
   453	
   454			w, u = fmtFrac(buf[:w], u, 9)
   455	
   456			// u is now integer seconds
   457			w = fmtInt(buf[:w], u%60)
   458			u /= 60
   459	
   460			// u is now integer minutes
   461			if u > 0 {
   462				w--
   463				buf[w] = 'm'
   464				w = fmtInt(buf[:w], u%60)
   465				u /= 60
   466	
   467				// u is now integer hours
   468				// Stop at hours because days can be different lengths.
   469				if u > 0 {
   470					w--
   471					buf[w] = 'h'
   472					w = fmtInt(buf[:w], u)
   473				}
   474			}
   475		}
   476	
   477		if neg {
   478			w--
   479			buf[w] = '-'
   480		}
   481	
   482		return string(buf[w:])
   483	}
   484	
   485	// fmtFrac formats the fraction of v/10**prec (e.g., ".12345") into the
   486	// tail of buf, omitting trailing zeros.  it omits the decimal
   487	// point too when the fraction is 0.  It returns the index where the
   488	// output bytes begin and the value v/10**prec.
   489	func fmtFrac(buf []byte, v uint64, prec int) (nw int, nv uint64) {
   490		// Omit trailing zeros up to and including decimal point.
   491		w := len(buf)
   492		print := false
   493		for i := 0; i < prec; i++ {
   494			digit := v % 10
   495			print = print || digit != 0
   496			if print {
   497				w--
   498				buf[w] = byte(digit) + '0'
   499			}
   500			v /= 10
   501		}
   502		if print {
   503			w--
   504			buf[w] = '.'
   505		}
   506		return w, v
   507	}
   508	
   509	// fmtInt formats v into the tail of buf.
   510	// It returns the index where the output begins.
   511	func fmtInt(buf []byte, v uint64) int {
   512		w := len(buf)
   513		if v == 0 {
   514			w--
   515			buf[w] = '0'
   516		} else {
   517			for v > 0 {
   518				w--
   519				buf[w] = byte(v%10) + '0'
   520				v /= 10
   521			}
   522		}
   523		return w
   524	}
   525	
   526	// Nanoseconds returns the duration as an integer nanosecond count.
   527	func (d Duration) Nanoseconds() int64 { return int64(d) }
   528	
   529	// These methods return float64 because the dominant
   530	// use case is for printing a floating point number like 1.5s, and
   531	// a truncation to integer would make them not useful in those cases.
   532	// Splitting the integer and fraction ourselves guarantees that
   533	// converting the returned float64 to an integer rounds the same
   534	// way that a pure integer conversion would have, even in cases
   535	// where, say, float64(d.Nanoseconds())/1e9 would have rounded
   536	// differently.
   537	
   538	// Seconds returns the duration as a floating point number of seconds.
   539	func (d Duration) Seconds() float64 {
   540		sec := d / Second
   541		nsec := d % Second
   542		return float64(sec) + float64(nsec)*1e-9
   543	}
   544	
   545	// Minutes returns the duration as a floating point number of minutes.
   546	func (d Duration) Minutes() float64 {
   547		min := d / Minute
   548		nsec := d % Minute
   549		return float64(min) + float64(nsec)*(1e-9/60)
   550	}
   551	
   552	// Hours returns the duration as a floating point number of hours.
   553	func (d Duration) Hours() float64 {
   554		hour := d / Hour
   555		nsec := d % Hour
   556		return float64(hour) + float64(nsec)*(1e-9/60/60)
   557	}
   558	
   559	// Add returns the time t+d.
   560	func (t Time) Add(d Duration) Time {
   561		t.sec += int64(d / 1e9)
   562		t.nsec += int32(d % 1e9)
   563		if t.nsec >= 1e9 {
   564			t.sec++
   565			t.nsec -= 1e9
   566		} else if t.nsec < 0 {
   567			t.sec--
   568			t.nsec += 1e9
   569		}
   570		return t
   571	}
   572	
   573	// Sub returns the duration t-u.
   574	// To compute t-d for a duration d, use t.Add(-d).
   575	func (t Time) Sub(u Time) Duration {
   576		return Duration(t.sec-u.sec)*Second + Duration(t.nsec-u.nsec)
   577	}
   578	
   579	// Since returns the time elapsed since t.
   580	// It is shorthand for time.Now().Sub(t).
   581	func Since(t Time) Duration {
   582		return Now().Sub(t)
   583	}
   584	
   585	// AddDate returns the time corresponding to adding the
   586	// given number of years, months, and days to t.
   587	// For example, AddDate(-1, 2, 3) applied to January 1, 2011
   588	// returns March 4, 2010.
   589	//
   590	// AddDate normalizes its result in the same way that Date does,
   591	// so, for example, adding one month to October 31 yields
   592	// December 1, the normalized form for November 31.
   593	func (t Time) AddDate(years int, months int, days int) Time {
   594		year, month, day := t.Date()
   595		hour, min, sec := t.Clock()
   596		return Date(year+years, month+Month(months), day+days, hour, min, sec, int(t.nsec), t.loc)
   597	}
   598	
   599	const (
   600		secondsPerMinute = 60
   601		secondsPerHour   = 60 * 60
   602		secondsPerDay    = 24 * secondsPerHour
   603		secondsPerWeek   = 7 * secondsPerDay
   604		daysPer400Years  = 365*400 + 97
   605		daysPer100Years  = 365*100 + 24
   606		daysPer4Years    = 365*4 + 1
   607		days1970To2001   = 31*365 + 8
   608	)
   609	
   610	// date computes the year and, only when full=true,
   611	// the month and day in which t occurs.
   612	func (t Time) date(full bool) (year int, month Month, day int, yday int) {
   613		// Split into time and day.
   614		d := t.abs() / secondsPerDay
   615	
   616		// Account for 400 year cycles.
   617		n := d / daysPer400Years
   618		y := 400 * n
   619		d -= daysPer400Years * n
   620	
   621		// Cut off 100-year cycles.
   622		// The last cycle has one extra leap year, so on the last day
   623		// of that year, day / daysPer100Years will be 4 instead of 3.
   624		// Cut it back down to 3 by subtracting n>>2.
   625		n = d / daysPer100Years
   626		n -= n >> 2
   627		y += 100 * n
   628		d -= daysPer100Years * n
   629	
   630		// Cut off 4-year cycles.
   631		// The last cycle has a missing leap year, which does not
   632		// affect the computation.
   633		n = d / daysPer4Years
   634		y += 4 * n
   635		d -= daysPer4Years * n
   636	
   637		// Cut off years within a 4-year cycle.
   638		// The last year is a leap year, so on the last day of that year,
   639		// day / 365 will be 4 instead of 3.  Cut it back down to 3
   640		// by subtracting n>>2.
   641		n = d / 365
   642		n -= n >> 2
   643		y += n
   644		d -= 365 * n
   645	
   646		year = int(int64(y) + absoluteZeroYear)
   647		yday = int(d)
   648	
   649		if !full {
   650			return
   651		}
   652	
   653		day = yday
   654		if isLeap(year) {
   655			// Leap year
   656			switch {
   657			case day > 31+29-1:
   658				// After leap day; pretend it wasn't there.
   659				day--
   660			case day == 31+29-1:
   661				// Leap day.
   662				month = February
   663				day = 29
   664				return
   665			}
   666		}
   667	
   668		// Estimate month on assumption that every month has 31 days.
   669		// The estimate may be too low by at most one month, so adjust.
   670		month = Month(day / 31)
   671		end := int(daysBefore[month+1])
   672		var begin int
   673		if day >= end {
   674			month++
   675			begin = end
   676		} else {
   677			begin = int(daysBefore[month])
   678		}
   679	
   680		month++ // because January is 1
   681		day = day - begin + 1
   682		return
   683	}
   684	
   685	// daysBefore[m] counts the number of days in a non-leap year
   686	// before month m begins.  There is an entry for m=12, counting
   687	// the number of days before January of next year (365).
   688	var daysBefore = [...]int32{
   689		0,
   690		31,
   691		31 + 28,
   692		31 + 28 + 31,
   693		31 + 28 + 31 + 30,
   694		31 + 28 + 31 + 30 + 31,
   695		31 + 28 + 31 + 30 + 31 + 30,
   696		31 + 28 + 31 + 30 + 31 + 30 + 31,
   697		31 + 28 + 31 + 30 + 31 + 30 + 31 + 31,
   698		31 + 28 + 31 + 30 + 31 + 30 + 31 + 31 + 30,
   699		31 + 28 + 31 + 30 + 31 + 30 + 31 + 31 + 30 + 31,
   700		31 + 28 + 31 + 30 + 31 + 30 + 31 + 31 + 30 + 31 + 30,
   701		31 + 28 + 31 + 30 + 31 + 30 + 31 + 31 + 30 + 31 + 30 + 31,
   702	}
   703	
   704	func daysIn(m Month, year int) int {
   705		if m == February && isLeap(year) {
   706			return 29
   707		}
   708		return int(daysBefore[m] - daysBefore[m-1])
   709	}
   710	
   711	// Provided by package runtime.
   712	func now() (sec int64, nsec int32)
   713	
   714	// Now returns the current local time.
   715	func Now() Time {
   716		sec, nsec := now()
   717		return Time{sec + unixToInternal, nsec, Local}
   718	}
   719	
   720	// UTC returns t with the location set to UTC.
   721	func (t Time) UTC() Time {
   722		t.loc = UTC
   723		return t
   724	}
   725	
   726	// Local returns t with the location set to local time.
   727	func (t Time) Local() Time {
   728		t.loc = Local
   729		return t
   730	}
   731	
   732	// In returns t with the location information set to loc.
   733	//
   734	// In panics if loc is nil.
   735	func (t Time) In(loc *Location) Time {
   736		if loc == nil {
   737			panic("time: missing Location in call to Time.In")
   738		}
   739		t.loc = loc
   740		return t
   741	}
   742	
   743	// Location returns the time zone information associated with t.
   744	func (t Time) Location() *Location {
   745		l := t.loc
   746		if l == nil {
   747			l = UTC
   748		}
   749		return l
   750	}
   751	
   752	// Zone computes the time zone in effect at time t, returning the abbreviated
   753	// name of the zone (such as "CET") and its offset in seconds east of UTC.
   754	func (t Time) Zone() (name string, offset int) {
   755		name, offset, _, _, _ = t.loc.lookup(t.sec + internalToUnix)
   756		return
   757	}
   758	
   759	// Unix returns t as a Unix time, the number of seconds elapsed
   760	// since January 1, 1970 UTC.
   761	func (t Time) Unix() int64 {
   762		return t.sec + internalToUnix
   763	}
   764	
   765	// UnixNano returns t as a Unix time, the number of nanoseconds elapsed
   766	// since January 1, 1970 UTC. The result is undefined if the Unix time
   767	// in nanoseconds cannot be represented by an int64. Note that this
   768	// means the result of calling UnixNano on the zero Time is undefined.
   769	func (t Time) UnixNano() int64 {
   770		return (t.sec+internalToUnix)*1e9 + int64(t.nsec)
   771	}
   772	
   773	const timeGobVersion byte = 1
   774	
   775	// GobEncode implements the gob.GobEncoder interface.
   776	func (t Time) GobEncode() ([]byte, error) {
   777		var offsetMin int16 // minutes east of UTC. -1 is UTC.
   778	
   779		if t.Location() == &utcLoc {
   780			offsetMin = -1
   781		} else {
   782			_, offset := t.Zone()
   783			if offset%60 != 0 {
   784				return nil, errors.New("Time.GobEncode: zone offset has fractional minute")
   785			}
   786			offset /= 60
   787			if offset < -32768 || offset == -1 || offset > 32767 {
   788				return nil, errors.New("Time.GobEncode: unexpected zone offset")
   789			}
   790			offsetMin = int16(offset)
   791		}
   792	
   793		enc := []byte{
   794			timeGobVersion,    // byte 0 : version
   795			byte(t.sec >> 56), // bytes 1-8: seconds
   796			byte(t.sec >> 48),
   797			byte(t.sec >> 40),
   798			byte(t.sec >> 32),
   799			byte(t.sec >> 24),
   800			byte(t.sec >> 16),
   801			byte(t.sec >> 8),
   802			byte(t.sec),
   803			byte(t.nsec >> 24), // bytes 9-12: nanoseconds
   804			byte(t.nsec >> 16),
   805			byte(t.nsec >> 8),
   806			byte(t.nsec),
   807			byte(offsetMin >> 8), // bytes 13-14: zone offset in minutes
   808			byte(offsetMin),
   809		}
   810	
   811		return enc, nil
   812	}
   813	
   814	// GobDecode implements the gob.GobDecoder interface.
   815	func (t *Time) GobDecode(buf []byte) error {
   816		if len(buf) == 0 {
   817			return errors.New("Time.GobDecode: no data")
   818		}
   819	
   820		if buf[0] != timeGobVersion {
   821			return errors.New("Time.GobDecode: unsupported version")
   822		}
   823	
   824		if len(buf) != /*version*/ 1+ /*sec*/ 8+ /*nsec*/ 4+ /*zone offset*/ 2 {
   825			return errors.New("Time.GobDecode: invalid length")
   826		}
   827	
   828		buf = buf[1:]
   829		t.sec = int64(buf[7]) | int64(buf[6])<<8 | int64(buf[5])<<16 | int64(buf[4])<<24 |
   830			int64(buf[3])<<32 | int64(buf[2])<<40 | int64(buf[1])<<48 | int64(buf[0])<<56
   831	
   832		buf = buf[8:]
   833		t.nsec = int32(buf[3]) | int32(buf[2])<<8 | int32(buf[1])<<16 | int32(buf[0])<<24
   834	
   835		buf = buf[4:]
   836		offset := int(int16(buf[1])|int16(buf[0])<<8) * 60
   837	
   838		if offset == -1*60 {
   839			t.loc = &utcLoc
   840		} else if _, localoff, _, _, _ := Local.lookup(t.sec + internalToUnix); offset == localoff {
   841			t.loc = Local
   842		} else {
   843			t.loc = FixedZone("", offset)
   844		}
   845	
   846		return nil
   847	}
   848	
   849	// MarshalJSON implements the json.Marshaler interface.
   850	// Time is formatted as RFC3339.
   851	func (t Time) MarshalJSON() ([]byte, error) {
   852		if y := t.Year(); y < 0 || y >= 10000 {
   853			return nil, errors.New("Time.MarshalJSON: year outside of range [0,9999]")
   854		}
   855		return []byte(t.Format(`"` + RFC3339Nano + `"`)), nil
   856	}
   857	
   858	// UnmarshalJSON implements the json.Unmarshaler interface.
   859	// Time is expected in RFC3339 format.
   860	func (t *Time) UnmarshalJSON(data []byte) (err error) {
   861		// Fractional seconds are handled implicitly by Parse.
   862		*t, err = Parse(`"`+RFC3339+`"`, string(data))
   863		return
   864	}
   865	
   866	// Unix returns the local Time corresponding to the given Unix time,
   867	// sec seconds and nsec nanoseconds since January 1, 1970 UTC.
   868	// It is valid to pass nsec outside the range [0, 999999999].
   869	func Unix(sec int64, nsec int64) Time {
   870		if nsec < 0 || nsec >= 1e9 {
   871			n := nsec / 1e9
   872			sec += n
   873			nsec -= n * 1e9
   874			if nsec < 0 {
   875				nsec += 1e9
   876				sec--
   877			}
   878		}
   879		return Time{sec + unixToInternal, int32(nsec), Local}
   880	}
   881	
   882	func isLeap(year int) bool {
   883		return year%4 == 0 && (year%100 != 0 || year%400 == 0)
   884	}
   885	
   886	// norm returns nhi, nlo such that
   887	//	hi * base + lo == nhi * base + nlo
   888	//	0 <= nlo < base
   889	func norm(hi, lo, base int) (nhi, nlo int) {
   890		if lo < 0 {
   891			n := (-lo-1)/base + 1
   892			hi -= n
   893			lo += n * base
   894		}
   895		if lo >= base {
   896			n := lo / base
   897			hi += n
   898			lo -= n * base
   899		}
   900		return hi, lo
   901	}
   902	
   903	// Date returns the Time corresponding to
   904	//	yyyy-mm-dd hh:mm:ss + nsec nanoseconds
   905	// in the appropriate zone for that time in the given location.
   906	//
   907	// The month, day, hour, min, sec, and nsec values may be outside
   908	// their usual ranges and will be normalized during the conversion.
   909	// For example, October 32 converts to November 1.
   910	//
   911	// A daylight savings time transition skips or repeats times.
   912	// For example, in the United States, March 13, 2011 2:15am never occurred,
   913	// while November 6, 2011 1:15am occurred twice.  In such cases, the
   914	// choice of time zone, and therefore the time, is not well-defined.
   915	// Date returns a time that is correct in one of the two zones involved
   916	// in the transition, but it does not guarantee which.
   917	//
   918	// Date panics if loc is nil.
   919	func Date(year int, month Month, day, hour, min, sec, nsec int, loc *Location) Time {
   920		if loc == nil {
   921			panic("time: missing Location in call to Date")
   922		}
   923	
   924		// Normalize month, overflowing into year.
   925		m := int(month) - 1
   926		year, m = norm(year, m, 12)
   927		month = Month(m) + 1
   928	
   929		// Normalize nsec, sec, min, hour, overflowing into day.
   930		sec, nsec = norm(sec, nsec, 1e9)
   931		min, sec = norm(min, sec, 60)
   932		hour, min = norm(hour, min, 60)
   933		day, hour = norm(day, hour, 24)
   934	
   935		y := uint64(int64(year) - absoluteZeroYear)
   936	
   937		// Compute days since the absolute epoch.
   938	
   939		// Add in days from 400-year cycles.
   940		n := y / 400
   941		y -= 400 * n
   942		d := daysPer400Years * n
   943	
   944		// Add in 100-year cycles.
   945		n = y / 100
   946		y -= 100 * n
   947		d += daysPer100Years * n
   948	
   949		// Add in 4-year cycles.
   950		n = y / 4
   951		y -= 4 * n
   952		d += daysPer4Years * n
   953	
   954		// Add in non-leap years.
   955		n = y
   956		d += 365 * n
   957	
   958		// Add in days before this month.
   959		d += uint64(daysBefore[month-1])
   960		if isLeap(year) && month >= March {
   961			d++ // February 29
   962		}
   963	
   964		// Add in days before today.
   965		d += uint64(day - 1)
   966	
   967		// Add in time elapsed today.
   968		abs := d * secondsPerDay
   969		abs += uint64(hour*secondsPerHour + min*secondsPerMinute + sec)
   970	
   971		unix := int64(abs) + (absoluteToInternal + internalToUnix)
   972	
   973		// Look for zone offset for t, so we can adjust to UTC.
   974		// The lookup function expects UTC, so we pass t in the
   975		// hope that it will not be too close to a zone transition,
   976		// and then adjust if it is.
   977		_, offset, _, start, end := loc.lookup(unix)
   978		if offset != 0 {
   979			switch utc := unix - int64(offset); {
   980			case utc < start:
   981				_, offset, _, _, _ = loc.lookup(start - 1)
   982			case utc >= end:
   983				_, offset, _, _, _ = loc.lookup(end)
   984			}
   985			unix -= int64(offset)
   986		}
   987	
   988		return Time{unix + unixToInternal, int32(nsec), loc}
   989	}