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// Copyright 2009 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

// Package time provides functionality for measuring and displaying time.
//
// The calendrical calculations always assume a Gregorian calendar.
package time

import "errors"

// A Time represents an instant in time with nanosecond precision.
//
// Programs using times should typically store and pass them as values,
// not pointers.  That is, time variables and struct fields should be of
// type time.Time, not *time.Time.
//
// Time instants can be compared using the Before, After, and Equal methods.
// The Sub method subtracts two instants, producing a Duration.
// The Add method adds a Time and a Duration, producing a Time.
//
// The zero value of type Time is January 1, year 1, 00:00:00.000000000 UTC.
// As this time is unlikely to come up in practice, the IsZero method gives
// a simple way of detecting a time that has not been initialized explicitly.
//
// Each Time has associated with it a Location, consulted when computing the
// presentation form of the time, such as in the Format, Hour, and Year methods.
// The methods Local, UTC, and In return a Time with a specific location.
// Changing the location in this way changes only the presentation; it does not
// change the instant in time being denoted and therefore does not affect the
// computations described in earlier paragraphs.
//
type Time struct {
        // sec gives the number of seconds elapsed since
        // January 1, year 1 00:00:00 UTC.
        sec int64

        // nsec specifies a non-negative nanosecond
        // offset within the second named by Seconds.
        // It must be in the range [0, 999999999].
        nsec int32

        // loc specifies the Location that should be used to
        // determine the minute, hour, month, day, and year
        // that correspond to this Time.
        // Only the zero Time has a nil Location.
        // In that case it is interpreted to mean UTC.
        loc *Location
}

// After reports whether the time instant t is after u.
func (t Time) After(u Time) bool {
        return t.sec > u.sec || t.sec == u.sec && t.nsec > u.nsec
}

// Before reports whether the time instant t is before u.
func (t Time) Before(u Time) bool {
        return t.sec < u.sec || t.sec == u.sec && t.nsec < u.nsec
}

// Equal reports whether t and u represent the same time instant.
// Two times can be equal even if they are in different locations.
// For example, 6:00 +0200 CEST and 4:00 UTC are Equal.
// This comparison is different from using t == u, which also compares
// the locations.
func (t Time) Equal(u Time) bool {
        return t.sec == u.sec && t.nsec == u.nsec
}

// A Month specifies a month of the year (January = 1, ...).
type Month int

const (
        January Month = 1 + iota
        February
        March
        April
        May
        June
        July
        August
        September
        October
        November
        December
)

var months = [...]string{
        "January",
        "February",
        "March",
        "April",
        "May",
        "June",
        "July",
        "August",
        "September",
        "October",
        "November",
        "December",
}

// String returns the English name of the month ("January", "February", ...).
func (m Month) String() string { return months[m-1] }

// A Weekday specifies a day of the week (Sunday = 0, ...).
type Weekday int

const (
        Sunday Weekday = iota
        Monday
        Tuesday
        Wednesday
        Thursday
        Friday
        Saturday
)

var days = [...]string{
        "Sunday",
        "Monday",
        "Tuesday",
        "Wednesday",
        "Thursday",
        "Friday",
        "Saturday",
}

// String returns the English name of the day ("Sunday", "Monday", ...).
func (d Weekday) String() string { return days[d] }

// Computations on time.
//
// The zero value for a Time is defined to be
//      January 1, year 1, 00:00:00.000000000 UTC
// which (1) looks like a zero, or as close as you can get in a date
// (1-1-1 00:00:00 UTC), (2) is unlikely enough to arise in practice to
// be a suitable "not set" sentinel, unlike Jan 1 1970, and (3) has a
// non-negative year even in time zones west of UTC, unlike 1-1-0
// 00:00:00 UTC, which would be 12-31-(-1) 19:00:00 in New York.
//
// The zero Time value does not force a specific epoch for the time
// representation.  For example, to use the Unix epoch internally, we
// could define that to distinguish a zero value from Jan 1 1970, that
// time would be represented by sec=-1, nsec=1e9.  However, it does
// suggest a representation, namely using 1-1-1 00:00:00 UTC as the
// epoch, and that's what we do.
//
// The Add and Sub computations are oblivious to the choice of epoch.
//
// The presentation computations - year, month, minute, and so on - all
// rely heavily on division and modulus by positive constants.  For
// calendrical calculations we want these divisions to round down, even
// for negative values, so that the remainder is always positive, but
// Go's division (like most hardware divison instructions) rounds to
// zero.  We can still do those computations and then adjust the result
// for a negative numerator, but it's annoying to write the adjustment
// over and over.  Instead, we can change to a different epoch so long
// ago that all the times we care about will be positive, and then round
// to zero and round down coincide.  These presentation routines already
// have to add the zone offset, so adding the translation to the
// alternate epoch is cheap.  For example, having a non-negative time t
// means that we can write
//
//      sec = t % 60
//
// instead of
//
//      sec = t % 60
//      if sec < 0 {
//              sec += 60
//      }
//
// everywhere.
//
// The calendar runs on an exact 400 year cycle: a 400-year calendar
// printed for 1970-2469 will apply as well to 2470-2869.  Even the days
// of the week match up.  It simplifies the computations to choose the
// cycle boundaries so that the exceptional years are always delayed as
// long as possible.  That means choosing a year equal to 1 mod 400, so
// that the first leap year is the 4th year, the first missed leap year
// is the 100th year, and the missed missed leap year is the 400th year.
// So we'd prefer instead to print a calendar for 2001-2400 and reuse it
// for 2401-2800.
//
// Finally, it's convenient if the delta between the Unix epoch and
// long-ago epoch is representable by an int64 constant.
//
// These three considerations—choose an epoch as early as possible, that
// uses a year equal to 1 mod 400, and that is no more than 2⁶³ seconds
// earlier than 1970—bring us to the year -292277022399.  We refer to
// this year as the absolute zero year, and to times measured as a uint64
// seconds since this year as absolute times.
//
// Times measured as an int64 seconds since the year 1—the representation
// used for Time's sec field—are called internal times.
//
// Times measured as an int64 seconds since the year 1970 are called Unix
// times.
//
// It is tempting to just use the year 1 as the absolute epoch, defining
// that the routines are only valid for years >= 1.  However, the
// routines would then be invalid when displaying the epoch in time zones
// west of UTC, since it is year 0.  It doesn't seem tenable to say that
// printing the zero time correctly isn't supported in half the time
// zones.  By comparison, it's reasonable to mishandle some times in
// the year -292277022399.
//
// All this is opaque to clients of the API and can be changed if a
// better implementation presents itself.

const (
        // The unsigned zero year for internal calculations.
        // Must be 1 mod 400, and times before it will not compute correctly,
        // but otherwise can be changed at will.
        absoluteZeroYear = -292277022399

        // The year of the zero Time.
        // Assumed by the unixToInternal computation below.
        internalYear = 1

        // The year of the zero Unix time.
        unixYear = 1970

        // Offsets to convert between internal and absolute or Unix times.
        absoluteToInternal int64 = (absoluteZeroYear - internalYear) * 365.2425 * secondsPerDay
        internalToAbsolute       = -absoluteToInternal

        unixToInternal int64 = (1969*365 + 1969/4 - 1969/100 + 1969/400) * secondsPerDay
        internalToUnix int64 = -unixToInternal
)

// IsZero reports whether t represents the zero time instant,
// January 1, year 1, 00:00:00 UTC.
func (t Time) IsZero() bool {
        return t.sec == 0 && t.nsec == 0
}

// abs returns the time t as an absolute time, adjusted by the zone offset.
// It is called when computing a presentation property like Month or Hour.
func (t Time) abs() uint64 {
        l := t.loc
        if l == nil {
                l = &utcLoc
        }
        // Avoid function call if we hit the local time cache.
        sec := t.sec + internalToUnix
        if l != &utcLoc {
                if l.cacheZone != nil && l.cacheStart <= sec && sec < l.cacheEnd {
                        sec += int64(l.cacheZone.offset)
                } else {
                        _, offset, _, _, _ := l.lookup(sec)
                        sec += int64(offset)
                }
        }
        return uint64(sec + (unixToInternal + internalToAbsolute))
}

// Date returns the year, month, and day in which t occurs.
func (t Time) Date() (year int, month Month, day int) {
        year, month, day, _ = t.date(true)
        return
}

// Year returns the year in which t occurs.
func (t Time) Year() int {
        year, _, _, _ := t.date(false)
        return year
}

// Month returns the month of the year specified by t.
func (t Time) Month() Month {
        _, month, _, _ := t.date(true)
        return month
}

// Day returns the day of the month specified by t.
func (t Time) Day() int {
        _, _, day, _ := t.date(true)
        return day
}

// Weekday returns the day of the week specified by t.
func (t Time) Weekday() Weekday {
        // January 1 of the absolute year, like January 1 of 2001, was a Monday.
        sec := (t.abs() + uint64(Monday)*secondsPerDay) % secondsPerWeek
        return Weekday(int(sec) / secondsPerDay)
}

// ISOWeek returns the ISO 8601 year and week number in which t occurs.
// Week ranges from 1 to 53. Jan 01 to Jan 03 of year n might belong to
// week 52 or 53 of year n-1, and Dec 29 to Dec 31 might belong to week 1
// of year n+1.
func (t Time) ISOWeek() (year, week int) {
        year, month, day, yday := t.date(true)
        wday := int(t.Weekday()+6) % 7 // weekday but Monday = 0.
        const (
                Mon int = iota
                Tue
                Wed
                Thu
                Fri
                Sat
                Sun
        )

        // Calculate week as number of Mondays in year up to
        // and including today, plus 1 because the first week is week 0.
        // Putting the + 1 inside the numerator as a + 7 keeps the
        // numerator from being negative, which would cause it to
        // round incorrectly.
        week = (yday - wday + 7) / 7

        // The week number is now correct under the assumption
        // that the first Monday of the year is in week 1.
        // If Jan 1 is a Tuesday, Wednesday, or Thursday, the first Monday
        // is actually in week 2.
        jan1wday := (wday - yday + 7*53) % 7
        if Tue <= jan1wday && jan1wday <= Thu {
                week++
        }

        // If the week number is still 0, we're in early January but in
        // the last week of last year.
        if week == 0 {
                year--
                week = 52
                // A year has 53 weeks when Jan 1 or Dec 31 is a Thursday,
                // meaning Jan 1 of the next year is a Friday
                // or it was a leap year and Jan 1 of the next year is a Saturday.
                if jan1wday == Fri || (jan1wday == Sat && isLeap(year)) {
                        week++
                }
        }

        // December 29 to 31 are in week 1 of next year if
        // they are after the last Thursday of the year and
        // December 31 is a Monday, Tuesday, or Wednesday.
        if month == December && day >= 29 && wday < Thu {
                if dec31wday := (wday + 31 - day) % 7; Mon <= dec31wday && dec31wday <= Wed {
                        year++
                        week = 1
                }
        }

        return
}

// Clock returns the hour, minute, and second within the day specified by t.
func (t Time) Clock() (hour, min, sec int) {
        sec = int(t.abs() % secondsPerDay)
        hour = sec / secondsPerHour
        sec -= hour * secondsPerHour
        min = sec / secondsPerMinute
        sec -= min * secondsPerMinute
        return
}

// Hour returns the hour within the day specified by t, in the range [0, 23].
func (t Time) Hour() int {
        return int(t.abs()%secondsPerDay) / secondsPerHour
}

// Minute returns the minute offset within the hour specified by t, in the range [0, 59].
func (t Time) Minute() int {
        return int(t.abs()%secondsPerHour) / secondsPerMinute
}

// Second returns the second offset within the minute specified by t, in the range [0, 59].
func (t Time) Second() int {
        return int(t.abs() % secondsPerMinute)
}

// Nanosecond returns the nanosecond offset within the second specified by t,
// in the range [0, 999999999].
func (t Time) Nanosecond() int {
        return int(t.nsec)
}

// A Duration represents the elapsed time between two instants
// as an int64 nanosecond count.  The representation limits the
// largest representable duration to approximately 290 years.
type Duration int64

// Common durations.  There is no definition for units of Day or larger
// to avoid confusion across daylight savings time zone transitions.
const (
        Nanosecond  Duration = 1
        Microsecond          = 1000 * Nanosecond
        Millisecond          = 1000 * Microsecond
        Second               = 1000 * Millisecond
        Minute               = 60 * Second
        Hour                 = 60 * Minute
)

// Duration returns a string representing the duration in the form "72h3m0.5s".
// Leading zero units are omitted.  As a special case, durations less than one
// second format use a smaller unit (milli-, micro-, or nanoseconds) to ensure
// that the leading digit is non-zero.  The zero duration formats as 0,
// with no unit.
func (d Duration) String() string {
        // Largest time is 2540400h10m10.000000000s
        var buf [32]byte
        w := len(buf)

        u := uint64(d)
        neg := d < 0
        if neg {
                u = -u
        }

        if u < uint64(Second) {
                // Special case: if duration is smaller than a second,
                // use smaller units, like 1.2ms
                var (
                        prec int
                        unit byte
                )
                switch {
                case u == 0:
                        return "0"
                case u < uint64(Microsecond):
                        // print nanoseconds
                        prec = 0
                        unit = 'n'
                case u < uint64(Millisecond):
                        // print microseconds
                        prec = 3
                        unit = 'u'
                default:
                        // print milliseconds
                        prec = 6
                        unit = 'm'
                }
                w -= 2
                buf[w] = unit
                buf[w+1] = 's'
                w, u = fmtFrac(buf[:w], u, prec)
                w = fmtInt(buf[:w], u)
        } else {
                w--
                buf[w] = 's'

                w, u = fmtFrac(buf[:w], u, 9)

                // u is now integer seconds
                w = fmtInt(buf[:w], u%60)
                u /= 60

                // u is now integer minutes
                if u > 0 {
                        w--
                        buf[w] = 'm'
                        w = fmtInt(buf[:w], u%60)
                        u /= 60

                        // u is now integer hours
                        // Stop at hours because days can be different lengths.
                        if u > 0 {
                                w--
                                buf[w] = 'h'
                                w = fmtInt(buf[:w], u)
                        }
                }
        }

        if neg {
                w--
                buf[w] = '-'
        }

        return string(buf[w:])
}

// fmtFrac formats the fraction of v/10**prec (e.g., ".12345") into the
// tail of buf, omitting trailing zeros.  it omits the decimal
// point too when the fraction is 0.  It returns the index where the
// output bytes begin and the value v/10**prec.
func fmtFrac(buf []byte, v uint64, prec int) (nw int, nv uint64) {
        // Omit trailing zeros up to and including decimal point.
        w := len(buf)
        print := false
        for i := 0; i < prec; i++ {
                digit := v % 10
                print = print || digit != 0
                if print {
                        w--
                        buf[w] = byte(digit) + '0'
                }
                v /= 10
        }
        if print {
                w--
                buf[w] = '.'
        }
        return w, v
}

// fmtInt formats v into the tail of buf.
// It returns the index where the output begins.
func fmtInt(buf []byte, v uint64) int {
        w := len(buf)
        if v == 0 {
                w--
                buf[w] = '0'
        } else {
                for v > 0 {
                        w--
                        buf[w] = byte(v%10) + '0'
                        v /= 10
                }
        }
        return w
}

// Nanoseconds returns the duration as an integer nanosecond count.
func (d Duration) Nanoseconds() int64 { return int64(d) }

// These methods return float64 because the dominant
// use case is for printing a floating point number like 1.5s, and
// a truncation to integer would make them not useful in those cases.
// Splitting the integer and fraction ourselves guarantees that
// converting the returned float64 to an integer rounds the same
// way that a pure integer conversion would have, even in cases
// where, say, float64(d.Nanoseconds())/1e9 would have rounded
// differently.

// Seconds returns the duration as a floating point number of seconds.
func (d Duration) Seconds() float64 {
        sec := d / Second
        nsec := d % Second
        return float64(sec) + float64(nsec)*1e-9
}

// Minutes returns the duration as a floating point number of minutes.
func (d Duration) Minutes() float64 {
        min := d / Minute
        nsec := d % Minute
        return float64(min) + float64(nsec)*(1e-9/60)
}

// Hours returns the duration as a floating point number of hours.
func (d Duration) Hours() float64 {
        hour := d / Hour
        nsec := d % Hour
        return float64(hour) + float64(nsec)*(1e-9/60/60)
}

// Add returns the time t+d.
func (t Time) Add(d Duration) Time {
        t.sec += int64(d / 1e9)
        t.nsec += int32(d % 1e9)
        if t.nsec >= 1e9 {
                t.sec++
                t.nsec -= 1e9
        } else if t.nsec < 0 {
                t.sec--
                t.nsec += 1e9
        }
        return t
}

// Sub returns the duration t-u.
// To compute t-d for a duration d, use t.Add(-d).
func (t Time) Sub(u Time) Duration {
        return Duration(t.sec-u.sec)*Second + Duration(t.nsec-u.nsec)
}

// Since returns the time elapsed since t.
// It is shorthand for time.Now().Sub(t).
func Since(t Time) Duration {
        return Now().Sub(t)
}

// AddDate returns the time corresponding to adding the
// given number of years, months, and days to t.
// For example, AddDate(-1, 2, 3) applied to January 1, 2011
// returns March 4, 2010.
//
// AddDate normalizes its result in the same way that Date does,
// so, for example, adding one month to October 31 yields
// December 1, the normalized form for November 31.
func (t Time) AddDate(years int, months int, days int) Time {
        year, month, day := t.Date()
        hour, min, sec := t.Clock()
        return Date(year+years, month+Month(months), day+days, hour, min, sec, int(t.nsec), t.loc)
}

const (
        secondsPerMinute = 60
        secondsPerHour   = 60 * 60
        secondsPerDay    = 24 * secondsPerHour
        secondsPerWeek   = 7 * secondsPerDay
        daysPer400Years  = 365*400 + 97
        daysPer100Years  = 365*100 + 24
        daysPer4Years    = 365*4 + 1
        days1970To2001   = 31*365 + 8
)

// date computes the year and, only when full=true,
// the month and day in which t occurs.
func (t Time) date(full bool) (year int, month Month, day int, yday int) {
        // Split into time and day.
        d := t.abs() / secondsPerDay

        // Account for 400 year cycles.
        n := d / daysPer400Years
        y := 400 * n
        d -= daysPer400Years * n

        // Cut off 100-year cycles.
        // The last cycle has one extra leap year, so on the last day
        // of that year, day / daysPer100Years will be 4 instead of 3.
        // Cut it back down to 3 by subtracting n>>2.
        n = d / daysPer100Years
        n -= n >> 2
        y += 100 * n
        d -= daysPer100Years * n

        // Cut off 4-year cycles.
        // The last cycle has a missing leap year, which does not
        // affect the computation.
        n = d / daysPer4Years
        y += 4 * n
        d -= daysPer4Years * n

        // Cut off years within a 4-year cycle.
        // The last year is a leap year, so on the last day of that year,
        // day / 365 will be 4 instead of 3.  Cut it back down to 3
        // by subtracting n>>2.
        n = d / 365
        n -= n >> 2
        y += n
        d -= 365 * n

        year = int(int64(y) + absoluteZeroYear)
        yday = int(d)

        if !full {
                return
        }

        day = yday
        if isLeap(year) {
                // Leap year
                switch {
                case day > 31+29-1:
                        // After leap day; pretend it wasn't there.
                        day--
                case day == 31+29-1:
                        // Leap day.
                        month = February
                        day = 29
                        return
                }
        }

        // Estimate month on assumption that every month has 31 days.
        // The estimate may be too low by at most one month, so adjust.
        month = Month(day / 31)
        end := int(daysBefore[month+1])
        var begin int
        if day >= end {
                month++
                begin = end
        } else {
                begin = int(daysBefore[month])
        }

        month++ // because January is 1
        day = day - begin + 1
        return
}

// daysBefore[m] counts the number of days in a non-leap year
// before month m begins.  There is an entry for m=12, counting
// the number of days before January of next year (365).
var daysBefore = [...]int32{
        0,
        31,
        31 + 28,
        31 + 28 + 31,
        31 + 28 + 31 + 30,
        31 + 28 + 31 + 30 + 31,
        31 + 28 + 31 + 30 + 31 + 30,
        31 + 28 + 31 + 30 + 31 + 30 + 31,
        31 + 28 + 31 + 30 + 31 + 30 + 31 + 31,
        31 + 28 + 31 + 30 + 31 + 30 + 31 + 31 + 30,
        31 + 28 + 31 + 30 + 31 + 30 + 31 + 31 + 30 + 31,
        31 + 28 + 31 + 30 + 31 + 30 + 31 + 31 + 30 + 31 + 30,
        31 + 28 + 31 + 30 + 31 + 30 + 31 + 31 + 30 + 31 + 30 + 31,
}

func daysIn(m Month, year int) int {
        if m == February && isLeap(year) {
                return 29
        }
        return int(daysBefore[m] - daysBefore[m-1])
}

// Provided by package runtime.
func now() (sec int64, nsec int32)

// Now returns the current local time.
func Now() Time {
        sec, nsec := now()
        return Time{sec + unixToInternal, nsec, Local}
}

// UTC returns t with the location set to UTC.
func (t Time) UTC() Time {
        t.loc = UTC
        return t
}

// Local returns t with the location set to local time.
func (t Time) Local() Time {
        t.loc = Local
        return t
}

// In returns t with the location information set to loc.
//
// In panics if loc is nil.
func (t Time) In(loc *Location) Time {
        if loc == nil {
                panic("time: missing Location in call to Time.In")
        }
        t.loc = loc
        return t
}

// Location returns the time zone information associated with t.
func (t Time) Location() *Location {
        l := t.loc
        if l == nil {
                l = UTC
        }
        return l
}

// Zone computes the time zone in effect at time t, returning the abbreviated
// name of the zone (such as "CET") and its offset in seconds east of UTC.
func (t Time) Zone() (name string, offset int) {
        name, offset, _, _, _ = t.loc.lookup(t.sec + internalToUnix)
        return
}

// Unix returns the Unix time, the number of seconds elapsed
// since January 1, 1970 UTC.
func (t Time) Unix() int64 {
        return t.sec + internalToUnix
}

// UnixNano returns the Unix time, the number of nanoseconds elapsed
// since January 1, 1970 UTC.
func (t Time) UnixNano() int64 {
        return (t.sec+internalToUnix)*1e9 + int64(t.nsec)
}

type gobError string

func (g gobError) Error() string { return string(g) }

const timeGobVersion byte = 1

// GobEncode implements the gob.GobEncoder interface.
func (t Time) GobEncode() ([]byte, error) {
        var offsetMin int16 // minutes east of UTC. -1 is UTC.

        if t.Location() == &utcLoc {
                offsetMin = -1
        } else {
                _, offset := t.Zone()
                if offset%60 != 0 {
                        return nil, errors.New("Time.GobEncode: zone offset has fractional minute")
                }
                offset /= 60
                if offset < -32768 || offset == -1 || offset > 32767 {
                        return nil, errors.New("Time.GobEncode: unexpected zone offset")
                }
                offsetMin = int16(offset)
        }

        enc := []byte{
                timeGobVersion,    // byte 0 : version
                byte(t.sec >> 56), // bytes 1-8: seconds
                byte(t.sec >> 48),
                byte(t.sec >> 40),
                byte(t.sec >> 32),
                byte(t.sec >> 24),
                byte(t.sec >> 16),
                byte(t.sec >> 8),
                byte(t.sec),
                byte(t.nsec >> 24), // bytes 9-12: nanoseconds
                byte(t.nsec >> 16),
                byte(t.nsec >> 8),
                byte(t.nsec),
                byte(offsetMin >> 8), // bytes 13-14: zone offset in minutes
                byte(offsetMin),
        }

        return enc, nil
}

// GobDecode implements the gob.GobDecoder interface.
func (t *Time) GobDecode(buf []byte) error {
        if len(buf) == 0 {
                return errors.New("Time.GobDecode: no data")
        }

        if buf[0] != timeGobVersion {
                return errors.New("Time.GobDecode: unsupported version")
        }

        if len(buf) != /*version*/ 1+ /*sec*/ 8+ /*nsec*/ 4+ /*zone offset*/ 2 {
                return errors.New("Time.GobDecode: invalid length")
        }

        buf = buf[1:]
        t.sec = int64(buf[7]) | int64(buf[6])<<8 | int64(buf[5])<<16 | int64(buf[4])<<24 |
                int64(buf[3])<<32 | int64(buf[2])<<40 | int64(buf[1])<<48 | int64(buf[0])<<56

        buf = buf[8:]
        t.nsec = int32(buf[3]) | int32(buf[2])<<8 | int32(buf[1])<<16 | int32(buf[0])<<24

        buf = buf[4:]
        offset := int(int16(buf[1])|int16(buf[0])<<8) * 60

        if offset == -1*60 {
                t.loc = &utcLoc
        } else if _, localoff, _, _, _ := Local.lookup(t.sec + internalToUnix); offset == localoff {
                t.loc = Local
        } else {
                t.loc = FixedZone("", offset)
        }

        return nil
}

// MarshalJSON implements the json.Marshaler interface.
// Time is formatted as RFC3339.
func (t Time) MarshalJSON() ([]byte, error) {
        yearInt := t.Year()
        if yearInt < 0 || yearInt > 9999 {
                return nil, errors.New("Time.MarshalJSON: year outside of range [0,9999]")
        }

        // We need a four-digit year, but Format produces variable-width years.
        year := itoa(yearInt)
        year = "0000"[:4-len(year)] + year

        var formattedTime string
        if t.nsec == 0 {
                // RFC3339, no fractional second
                formattedTime = t.Format("-01-02T15:04:05Z07:00")
        } else {
                // RFC3339 with fractional second
                formattedTime = t.Format("-01-02T15:04:05.000000000Z07:00")

                // Trim trailing zeroes from fractional second.
                const nanoEnd = 24 // Index of last digit of fractional second
                var i int
                for i = nanoEnd; formattedTime[i] == '0'; i-- {
                        // Seek backwards until first significant digit is found.
                }

                formattedTime = formattedTime[:i+1] + formattedTime[nanoEnd+1:]
        }

        buf := make([]byte, 0, 1+len(year)+len(formattedTime)+1)
        buf = append(buf, '"')
        buf = append(buf, year...)
        buf = append(buf, formattedTime...)
        buf = append(buf, '"')
        return buf, nil
}

// UnmarshalJSON implements the json.Unmarshaler interface.
// Time is expected in RFC3339 format.
func (t *Time) UnmarshalJSON(data []byte) (err error) {
        *t, err = Parse("\""+RFC3339+"\"", string(data))
        // Fractional seconds are handled implicitly by Parse.
        return
}

// Unix returns the local Time corresponding to the given Unix time,
// sec seconds and nsec nanoseconds since January 1, 1970 UTC.
// It is valid to pass nsec outside the range [0, 999999999].
func Unix(sec int64, nsec int64) Time {
        if nsec < 0 || nsec >= 1e9 {
                n := nsec / 1e9
                sec += n
                nsec -= n * 1e9
                if nsec < 0 {
                        nsec += 1e9
                        sec--
                }
        }
        return Time{sec + unixToInternal, int32(nsec), Local}
}

func isLeap(year int) bool {
        return year%4 == 0 && (year%100 != 0 || year%400 == 0)
}

// norm returns nhi, nlo such that
//      hi * base + lo == nhi * base + nlo
//      0 <= nlo < base
func norm(hi, lo, base int) (nhi, nlo int) {
        if lo < 0 {
                n := (-lo-1)/base + 1
                hi -= n
                lo += n * base
        }
        if lo >= base {
                n := lo / base
                hi += n
                lo -= n * base
        }
        return hi, lo
}

// Date returns the Time corresponding to
//      yyyy-mm-dd hh:mm:ss + nsec nanoseconds
// in the appropriate zone for that time in the given location.
//
// The month, day, hour, min, sec, and nsec values may be outside
// their usual ranges and will be normalized during the conversion.
// For example, October 32 converts to November 1.
//
// A daylight savings time transition skips or repeats times.
// For example, in the United States, March 13, 2011 2:15am never occurred,
// while November 6, 2011 1:15am occurred twice.  In such cases, the
// choice of time zone, and therefore the time, is not well-defined.
// Date returns a time that is correct in one of the two zones involved
// in the transition, but it does not guarantee which.
//
// Date panics if loc is nil.
func Date(year int, month Month, day, hour, min, sec, nsec int, loc *Location) Time {
        if loc == nil {
                panic("time: missing Location in call to Date")
        }

        // Normalize month, overflowing into year.
        m := int(month) - 1
        year, m = norm(year, m, 12)
        month = Month(m) + 1

        // Normalize nsec, sec, min, hour, overflowing into day.
        sec, nsec = norm(sec, nsec, 1e9)
        min, sec = norm(min, sec, 60)
        hour, min = norm(hour, min, 60)
        day, hour = norm(day, hour, 24)

        y := uint64(int64(year) - absoluteZeroYear)

        // Compute days since the absolute epoch.

        // Add in days from 400-year cycles.
        n := y / 400
        y -= 400 * n
        d := daysPer400Years * n

        // Add in 100-year cycles.
        n = y / 100
        y -= 100 * n
        d += daysPer100Years * n

        // Add in 4-year cycles.
        n = y / 4
        y -= 4 * n
        d += daysPer4Years * n

        // Add in non-leap years.
        n = y
        d += 365 * n

        // Add in days before this month.
        d += uint64(daysBefore[month-1])
        if isLeap(year) && month >= March {
                d++ // February 29
        }

        // Add in days before today.
        d += uint64(day - 1)

        // Add in time elapsed today.
        abs := d * secondsPerDay
        abs += uint64(hour*secondsPerHour + min*secondsPerMinute + sec)

        unix := int64(abs) + (absoluteToInternal + internalToUnix)

        // Look for zone offset for t, so we can adjust to UTC.
        // The lookup function expects UTC, so we pass t in the
        // hope that it will not be too close to a zone transition,
        // and then adjust if it is.
        _, offset, _, start, end := loc.lookup(unix)
        if offset != 0 {
                switch utc := unix - int64(offset); {
                case utc < start:
                        _, offset, _, _, _ = loc.lookup(start - 1)
                case utc >= end:
                        _, offset, _, _, _ = loc.lookup(end)
                }
                unix -= int64(offset)
        }

        return Time{unix + unixToInternal, int32(nsec), loc}
}