src/time/zoneinfo.go - go - Git at Google

 // Copyright 2011 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

 import (
 	"errors"
 	"sync"
 	"syscall"
 )

 //go:generate env ZONEINFO=$GOROOT/lib/time/zoneinfo.zip go run genzabbrs.go -output zoneinfo_abbrs_windows.go

 // A Location maps time instants to the zone in use at that time.
 // Typically, the Location represents the collection of time offsets
 // in use in a geographical area. For many Locations the time offset varies
 // depending on whether daylight savings time is in use at the time instant.
 type Location struct {
 	name string
 	zone []zone
 	tx   []zoneTrans

 	// The tzdata information can be followed by a string that describes
 	// how to handle DST transitions not recorded in zoneTrans.
 	// The format is the TZ environment variable without a colon; see
 	// https://pubs.opengroup.org/onlinepubs/9699919799/basedefs/V1_chap08.html.
 	// Example string, for America/Los_Angeles: PST8PDT,M3.2.0,M11.1.0
 	extend string

 	// Most lookups will be for the current time.
 	// To avoid the binary search through tx, keep a
 	// static one-element cache that gives the correct
 	// zone for the time when the Location was created.
 	// if cacheStart <= t < cacheEnd,
 	// lookup can return cacheZone.
 	// The units for cacheStart and cacheEnd are seconds
 	// since January 1, 1970 UTC, to match the argument
 	// to lookup.
 	cacheStart int64
 	cacheEnd   int64
 	cacheZone  *zone
 }

 // A zone represents a single time zone such as CET.
 type zone struct {
 	name   string // abbreviated name, "CET"
 	offset int    // seconds east of UTC
 	isDST  bool   // is this zone Daylight Savings Time?
 }

 // A zoneTrans represents a single time zone transition.
 type zoneTrans struct {
 	when         int64 // transition time, in seconds since 1970 GMT
 	index        uint8 // the index of the zone that goes into effect at that time
 	isstd, isutc bool  // ignored - no idea what these mean
 }

 // alpha and omega are the beginning and end of time for zone
 // transitions.
 const (
 	alpha = -1 << 63  // math.MinInt64
 	omega = 1<<63 - 1 // math.MaxInt64
 )

 // UTC represents Universal Coordinated Time (UTC).
 var UTC *Location = &utcLoc

 // utcLoc is separate so that get can refer to &utcLoc
 // and ensure that it never returns a nil *Location,
 // even if a badly behaved client has changed UTC.
 var utcLoc = Location{name: "UTC"}

 // Local represents the system's local time zone.
 // On Unix systems, Local consults the TZ environment
 // variable to find the time zone to use. No TZ means
 // use the system default /etc/localtime.
 // TZ="" means use UTC.
 // TZ="foo" means use file foo in the system timezone directory.
 var Local *Location = &localLoc

 // localLoc is separate so that initLocal can initialize
 // it even if a client has changed Local.
 var localLoc Location
 var localOnce sync.Once

 func (l *Location) get() *Location {
 	if l == nil {
 		return &utcLoc
 	}
 	if l == &localLoc {
 		localOnce.Do(initLocal)
 	}
 	return l
 }

 // String returns a descriptive name for the time zone information,
 // corresponding to the name argument to LoadLocation or FixedZone.
 func (l *Location) String() string {
 	return l.get().name
 }

 var unnamedFixedZones []*Location
 var unnamedFixedZonesOnce sync.Once

 // FixedZone returns a Location that always uses
 // the given zone name and offset (seconds east of UTC).
 func FixedZone(name string, offset int) *Location {
 	// Most calls to FixedZone have an unnamed zone with an offset by the hour.
 	// Optimize for that case by returning the same *Location for a given hour.
 	const hoursBeforeUTC = 12
 	const hoursAfterUTC = 14
 	hour := offset / 60 / 60
 	if name == "" && -hoursBeforeUTC <= hour && hour <= +hoursAfterUTC && hour*60*60 == offset {
 		unnamedFixedZonesOnce.Do(func() {
 			unnamedFixedZones = make([]*Location, hoursBeforeUTC+1+hoursAfterUTC)
 			for hr := -hoursBeforeUTC; hr <= +hoursAfterUTC; hr++ {
 				unnamedFixedZones[hr+hoursBeforeUTC] = fixedZone("", hr*60*60)
 			}
 		})
 		return unnamedFixedZones[hour+hoursBeforeUTC]
 	}
 	return fixedZone(name, offset)
 }

 func fixedZone(name string, offset int) *Location {
 	l := &Location{
 		name:       name,
 		zone:       []zone{{name, offset, false}},
 		tx:         []zoneTrans{{alpha, 0, false, false}},
 		cacheStart: alpha,
 		cacheEnd:   omega,
 	}
 	l.cacheZone = &l.zone[0]
 	return l
 }

 // lookup returns information about the time zone in use at an
 // instant in time expressed as seconds since January 1, 1970 00:00:00 UTC.
 //
 // The returned information gives the name of the zone (such as "CET"),
 // the start and end times bracketing sec when that zone is in effect,
 // the offset in seconds east of UTC (such as -5*60*60), and whether
 // the daylight savings is being observed at that time.
 func (l *Location) lookup(sec int64) (name string, offset int, start, end int64, isDST bool) {
 	l = l.get()

 	if len(l.zone) == 0 {
 		name = "UTC"
 		offset = 0
 		start = alpha
 		end = omega
 		isDST = false
 		return
 	}

 	if zone := l.cacheZone; zone != nil && l.cacheStart <= sec && sec < l.cacheEnd {
 		name = zone.name
 		offset = zone.offset
 		start = l.cacheStart
 		end = l.cacheEnd
 		isDST = zone.isDST
 		return
 	}

 	if len(l.tx) == 0 || sec < l.tx[0].when {
 		zone := &l.zone[l.lookupFirstZone()]
 		name = zone.name
 		offset = zone.offset
 		start = alpha
 		if len(l.tx) > 0 {
 			end = l.tx[0].when
 		} else {
 			end = omega
 		}
 		isDST = zone.isDST
 		return
 	}

 	// Binary search for entry with largest time <= sec.
 	// Not using sort.Search to avoid dependencies.
 	tx := l.tx
 	end = omega
 	lo := 0
 	hi := len(tx)
 	for hi-lo > 1 {
 		m := lo + (hi-lo)/2
 		lim := tx[m].when
 		if sec < lim {
 			end = lim
 			hi = m
 		} else {
 			lo = m
 		}
 	}
 	zone := &l.zone[tx[lo].index]
 	name = zone.name
 	offset = zone.offset
 	start = tx[lo].when
 	// end = maintained during the search
 	isDST = zone.isDST

 	// If we're at the end of the known zone transitions,
 	// try the extend string.
 	if lo == len(tx)-1 && l.extend != "" {
 		if ename, eoffset, estart, eend, eisDST, ok := tzset(l.extend, start, sec); ok {
 			return ename, eoffset, estart, eend, eisDST
 		}
 	}

 	return
 }

 // lookupFirstZone returns the index of the time zone to use for times
 // before the first transition time, or when there are no transition
 // times.
 //
 // The reference implementation in localtime.c from
 // https://www.iana.org/time-zones/repository/releases/tzcode2013g.tar.gz
 // implements the following algorithm for these cases:
 //  1. If the first zone is unused by the transitions, use it.
 //  2. Otherwise, if there are transition times, and the first
 //     transition is to a zone in daylight time, find the first
 //     non-daylight-time zone before and closest to the first transition
 //     zone.
 //  3. Otherwise, use the first zone that is not daylight time, if
 //     there is one.
 //  4. Otherwise, use the first zone.
 func (l *Location) lookupFirstZone() int {
 	// Case 1.
 	if !l.firstZoneUsed() {
 		return 0
 	}

 	// Case 2.
 	if len(l.tx) > 0 && l.zone[l.tx[0].index].isDST {
 		for zi := int(l.tx[0].index) - 1; zi >= 0; zi-- {
 			if !l.zone[zi].isDST {
 				return zi
 			}
 		}
 	}

 	// Case 3.
 	for zi := range l.zone {
 		if !l.zone[zi].isDST {
 			return zi
 		}
 	}

 	// Case 4.
 	return 0
 }

 // firstZoneUsed reports whether the first zone is used by some
 // transition.
 func (l *Location) firstZoneUsed() bool {
 	for _, tx := range l.tx {
 		if tx.index == 0 {
 			return true
 		}
 	}
 	return false
 }

 // tzset takes a timezone string like the one found in the TZ environment
 // variable, the time of the last time zone transition expressed as seconds
 // since January 1, 1970 00:00:00 UTC, and a time expressed the same way.
 // We call this a tzset string since in C the function tzset reads TZ.
 // The return values are as for lookup, plus ok which reports whether the
 // parse succeeded.
 func tzset(s string, lastTxSec, sec int64) (name string, offset int, start, end int64, isDST, ok bool) {
 	var (
 		stdName, dstName     string
 		stdOffset, dstOffset int
 	)

 	stdName, s, ok = tzsetName(s)
 	if ok {
 		stdOffset, s, ok = tzsetOffset(s)
 	}
 	if !ok {
 		return "", 0, 0, 0, false, false
 	}

 	// The numbers in the tzset string are added to local time to get UTC,
 	// but our offsets are added to UTC to get local time,
 	// so we negate the number we see here.
 	stdOffset = -stdOffset

 	if len(s) == 0 || s[0] == ',' {
 		// No daylight savings time.
 		return stdName, stdOffset, lastTxSec, omega, false, true
 	}

 	dstName, s, ok = tzsetName(s)
 	if ok {
 		if len(s) == 0 || s[0] == ',' {
 			dstOffset = stdOffset + secondsPerHour
 		} else {
 			dstOffset, s, ok = tzsetOffset(s)
 			dstOffset = -dstOffset // as with stdOffset, above
 		}
 	}
 	if !ok {
 		return "", 0, 0, 0, false, false
 	}

 	if len(s) == 0 {
 		// Default DST rules per tzcode.
 		s = ",M3.2.0,M11.1.0"
 	}
 	// The TZ definition does not mention ';' here but tzcode accepts it.
 	if s[0] != ',' && s[0] != ';' {
 		return "", 0, 0, 0, false, false
 	}
 	s = s[1:]

 	var startRule, endRule rule
 	startRule, s, ok = tzsetRule(s)
 	if !ok || len(s) == 0 || s[0] != ',' {
 		return "", 0, 0, 0, false, false
 	}
 	s = s[1:]
 	endRule, s, ok = tzsetRule(s)
 	if !ok || len(s) > 0 {
 		return "", 0, 0, 0, false, false
 	}

 	year, _, _, yday := absDate(uint64(sec+unixToInternal+internalToAbsolute), false)

 	ysec := int64(yday*secondsPerDay) + sec%secondsPerDay

 	// Compute start of year in seconds since Unix epoch.
 	d := daysSinceEpoch(year)
 	abs := int64(d * secondsPerDay)
 	abs += absoluteToInternal + internalToUnix

 	startSec := int64(tzruleTime(year, startRule, stdOffset))
 	endSec := int64(tzruleTime(year, endRule, dstOffset))
 	dstIsDST, stdIsDST := true, false
 	// Note: this is a flipping of "DST" and "STD" while retaining the labels
 	// This happens in southern hemispheres. The labelling here thus is a little
 	// inconsistent with the goal.
 	if endSec < startSec {
 		startSec, endSec = endSec, startSec
 		stdName, dstName = dstName, stdName
 		stdOffset, dstOffset = dstOffset, stdOffset
 		stdIsDST, dstIsDST = dstIsDST, stdIsDST
 	}

 	// The start and end values that we return are accurate
 	// close to a daylight savings transition, but are otherwise
 	// just the start and end of the year. That suffices for
 	// the only caller that cares, which is Date.
 	if ysec < startSec {
 		return stdName, stdOffset, abs, startSec + abs, stdIsDST, true
 	} else if ysec >= endSec {
 		return stdName, stdOffset, endSec + abs, abs + 365*secondsPerDay, stdIsDST, true
 	} else {
 		return dstName, dstOffset, startSec + abs, endSec + abs, dstIsDST, true
 	}
 }

 // tzsetName returns the timezone name at the start of the tzset string s,
 // and the remainder of s, and reports whether the parsing is OK.
 func tzsetName(s string) (string, string, bool) {
 	if len(s) == 0 {
 		return "", "", false
 	}
 	if s[0] != '<' {
 		for i, r := range s {
 			switch r {
 			case '0', '1', '2', '3', '4', '5', '6', '7', '8', '9', ',', '-', '+':
 				if i < 3 {
 					return "", "", false
 				}
 				return s[:i], s[i:], true
 			}
 		}
 		if len(s) < 3 {
 			return "", "", false
 		}
 		return s, "", true
 	} else {
 		for i, r := range s {
 			if r == '>' {
 				return s[1:i], s[i+1:], true
 			}
 		}
 		return "", "", false
 	}
 }

 // tzsetOffset returns the timezone offset at the start of the tzset string s,
 // and the remainder of s, and reports whether the parsing is OK.
 // The timezone offset is returned as a number of seconds.
 func tzsetOffset(s string) (offset int, rest string, ok bool) {
 	if len(s) == 0 {
 		return 0, "", false
 	}
 	neg := false
 	if s[0] == '+' {
 		s = s[1:]
 	} else if s[0] == '-' {
 		s = s[1:]
 		neg = true
 	}

 	// The tzdata code permits values up to 24 * 7 here,
 	// although POSIX does not.
 	var hours int
 	hours, s, ok = tzsetNum(s, 0, 24*7)
 	if !ok {
 		return 0, "", false
 	}
 	off := hours * secondsPerHour
 	if len(s) == 0 || s[0] != ':' {
 		if neg {
 			off = -off
 		}
 		return off, s, true
 	}

 	var mins int
 	mins, s, ok = tzsetNum(s[1:], 0, 59)
 	if !ok {
 		return 0, "", false
 	}
 	off += mins * secondsPerMinute
 	if len(s) == 0 || s[0] != ':' {
 		if neg {
 			off = -off
 		}
 		return off, s, true
 	}

 	var secs int
 	secs, s, ok = tzsetNum(s[1:], 0, 59)
 	if !ok {
 		return 0, "", false
 	}
 	off += secs

 	if neg {
 		off = -off
 	}
 	return off, s, true
 }

 // ruleKind is the kinds of rules that can be seen in a tzset string.
 type ruleKind int

 const (
 	ruleJulian ruleKind = iota
 	ruleDOY
 	ruleMonthWeekDay
 )

 // rule is a rule read from a tzset string.
 type rule struct {
 	kind ruleKind
 	day  int
 	week int
 	mon  int
 	time int // transition time
 }

 // tzsetRule parses a rule from a tzset string.
 // It returns the rule, and the remainder of the string, and reports success.
 func tzsetRule(s string) (rule, string, bool) {
 	var r rule
 	if len(s) == 0 {
 		return rule{}, "", false
 	}
 	ok := false
 	if s[0] == 'J' {
 		var jday int
 		jday, s, ok = tzsetNum(s[1:], 1, 365)
 		if !ok {
 			return rule{}, "", false
 		}
 		r.kind = ruleJulian
 		r.day = jday
 	} else if s[0] == 'M' {
 		var mon int
 		mon, s, ok = tzsetNum(s[1:], 1, 12)
 		if !ok || len(s) == 0 || s[0] != '.' {
 			return rule{}, "", false

 		}
 		var week int
 		week, s, ok = tzsetNum(s[1:], 1, 5)
 		if !ok || len(s) == 0 || s[0] != '.' {
 			return rule{}, "", false
 		}
 		var day int
 		day, s, ok = tzsetNum(s[1:], 0, 6)
 		if !ok {
 			return rule{}, "", false
 		}
 		r.kind = ruleMonthWeekDay
 		r.day = day
 		r.week = week
 		r.mon = mon
 	} else {
 		var day int
 		day, s, ok = tzsetNum(s, 0, 365)
 		if !ok {
 			return rule{}, "", false
 		}
 		r.kind = ruleDOY
 		r.day = day
 	}

 	if len(s) == 0 || s[0] != '/' {
 		r.time = 2 * secondsPerHour // 2am is the default
 		return r, s, true
 	}

 	offset, s, ok := tzsetOffset(s[1:])
 	if !ok {
 		return rule{}, "", false
 	}
 	r.time = offset

 	return r, s, true
 }

 // tzsetNum parses a number from a tzset string.
 // It returns the number, and the remainder of the string, and reports success.
 // The number must be between min and max.
 func tzsetNum(s string, min, max int) (num int, rest string, ok bool) {
 	if len(s) == 0 {
 		return 0, "", false
 	}
 	num = 0
 	for i, r := range s {
 		if r < '0' || r > '9' {
 			if i == 0 || num < min {
 				return 0, "", false
 			}
 			return num, s[i:], true
 		}
 		num *= 10
 		num += int(r) - '0'
 		if num > max {
 			return 0, "", false
 		}
 	}
 	if num < min {
 		return 0, "", false
 	}
 	return num, "", true
 }

 // tzruleTime takes a year, a rule, and a timezone offset,
 // and returns the number of seconds since the start of the year
 // that the rule takes effect.
 func tzruleTime(year int, r rule, off int) int {
 	var s int
 	switch r.kind {
 	case ruleJulian:
 		s = (r.day - 1) * secondsPerDay
 		if isLeap(year) && r.day >= 60 {
 			s += secondsPerDay
 		}
 	case ruleDOY:
 		s = r.day * secondsPerDay
 	case ruleMonthWeekDay:
 		// Zeller's Congruence.
 		m1 := (r.mon+9)%12 + 1
 		yy0 := year
 		if r.mon <= 2 {
 			yy0--
 		}
 		yy1 := yy0 / 100
 		yy2 := yy0 % 100
 		dow := ((26*m1-2)/10 + 1 + yy2 + yy2/4 + yy1/4 - 2*yy1) % 7
 		if dow < 0 {
 			dow += 7
 		}
 		// Now dow is the day-of-week of the first day of r.mon.
 		// Get the day-of-month of the first "dow" day.
 		d := r.day - dow
 		if d < 0 {
 			d += 7
 		}
 		for i := 1; i < r.week; i++ {
 			if d+7 >= daysIn(Month(r.mon), year) {
 				break
 			}
 			d += 7
 		}
 		d += int(daysBefore[r.mon-1])
 		if isLeap(year) && r.mon > 2 {
 			d++
 		}
 		s = d * secondsPerDay
 	}

 	return s + r.time - off
 }

 // lookupName returns information about the time zone with
 // the given name (such as "EST") at the given pseudo-Unix time
 // (what the given time of day would be in UTC).
 func (l *Location) lookupName(name string, unix int64) (offset int, ok bool) {
 	l = l.get()

 	// First try for a zone with the right name that was actually
 	// in effect at the given time. (In Sydney, Australia, both standard
 	// and daylight-savings time are abbreviated "EST". Using the
 	// offset helps us pick the right one for the given time.
 	// It's not perfect: during the backward transition we might pick
 	// either one.)
 	for i := range l.zone {
 		zone := &l.zone[i]
 		if zone.name == name {
 			nam, offset, _, _, _ := l.lookup(unix - int64(zone.offset))
 			if nam == zone.name {
 				return offset, true
 			}
 		}
 	}

 	// Otherwise fall back to an ordinary name match.
 	for i := range l.zone {
 		zone := &l.zone[i]
 		if zone.name == name {
 			return zone.offset, true
 		}
 	}

 	// Otherwise, give up.
 	return
 }

 // NOTE(rsc): Eventually we will need to accept the POSIX TZ environment
 // syntax too, but I don't feel like implementing it today.

 var errLocation = errors.New("time: invalid location name")

 var zoneinfo *string
 var zoneinfoOnce sync.Once

 // LoadLocation returns the Location with the given name.
 //
 // If the name is "" or "UTC", LoadLocation returns UTC.
 // If the name is "Local", LoadLocation returns Local.
 //
 // Otherwise, the name is taken to be a location name corresponding to a file
 // in the IANA Time Zone database, such as "America/New_York".
 //
 // LoadLocation looks for the IANA Time Zone database in the following
 // locations in order:
 //
 //   - the directory or uncompressed zip file named by the ZONEINFO environment variable
 //   - on a Unix system, the system standard installation location
 //   - $GOROOT/lib/time/zoneinfo.zip
 //   - the time/tzdata package, if it was imported
 func LoadLocation(name string) (*Location, error) {
 	if name == "" || name == "UTC" {
 		return UTC, nil
 	}
 	if name == "Local" {
 		return Local, nil
 	}
 	if containsDotDot(name) || name[0] == '/' || name[0] == '\\' {
 		// No valid IANA Time Zone name contains a single dot,
 		// much less dot dot. Likewise, none begin with a slash.
 		return nil, errLocation
 	}
 	zoneinfoOnce.Do(func() {
 		env, _ := syscall.Getenv("ZONEINFO")
 		zoneinfo = &env
 	})
 	var firstErr error
 	if *zoneinfo != "" {
 		if zoneData, err := loadTzinfoFromDirOrZip(*zoneinfo, name); err == nil {
 			if z, err := LoadLocationFromTZData(name, zoneData); err == nil {
 				return z, nil
 			}
 			firstErr = err
 		} else if err != syscall.ENOENT {
 			firstErr = err
 		}
 	}
 	if z, err := loadLocation(name, platformZoneSources); err == nil {
 		return z, nil
 	} else if firstErr == nil {
 		firstErr = err
 	}
 	return nil, firstErr
 }

 // containsDotDot reports whether s contains "..".
 func containsDotDot(s string) bool {
 	if len(s) < 2 {
 		return false
 	}
 	for i := 0; i < len(s)-1; i++ {
 		if s[i] == '.' && s[i+1] == '.' {
 			return true
 		}
 	}
 	return false
 }
	// Copyright 2011 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

	import (
	"errors"
	"sync"
	"syscall"
	)

	//go:generate env ZONEINFO=$GOROOT/lib/time/zoneinfo.zip go run genzabbrs.go -output zoneinfo_abbrs_windows.go

	// A Location maps time instants to the zone in use at that time.
	// Typically, the Location represents the collection of time offsets
	// in use in a geographical area. For many Locations the time offset varies
	// depending on whether daylight savings time is in use at the time instant.
	type Location struct {
	name string
	zone []zone
	tx []zoneTrans

	// The tzdata information can be followed by a string that describes
	// how to handle DST transitions not recorded in zoneTrans.
	// The format is the TZ environment variable without a colon; see
	// https://pubs.opengroup.org/onlinepubs/9699919799/basedefs/V1_chap08.html.
	// Example string, for America/Los_Angeles: PST8PDT,M3.2.0,M11.1.0
	extend string

	// Most lookups will be for the current time.
	// To avoid the binary search through tx, keep a
	// static one-element cache that gives the correct
	// zone for the time when the Location was created.
	// if cacheStart <= t < cacheEnd,
	// lookup can return cacheZone.
	// The units for cacheStart and cacheEnd are seconds
	// since January 1, 1970 UTC, to match the argument
	// to lookup.
	cacheStart int64
	cacheEnd int64
	cacheZone *zone
	}

	// A zone represents a single time zone such as CET.
	type zone struct {
	name string // abbreviated name, "CET"
	offset int // seconds east of UTC
	isDST bool // is this zone Daylight Savings Time?
	}

	// A zoneTrans represents a single time zone transition.
	type zoneTrans struct {
	when int64 // transition time, in seconds since 1970 GMT
	index uint8 // the index of the zone that goes into effect at that time
	isstd, isutc bool // ignored - no idea what these mean
	}

	// alpha and omega are the beginning and end of time for zone
	// transitions.
	const (
	alpha = -1 << 63 // math.MinInt64
	omega = 1<<63 - 1 // math.MaxInt64
	)

	// UTC represents Universal Coordinated Time (UTC).
	var UTC *Location = &utcLoc

	// utcLoc is separate so that get can refer to &utcLoc
	// and ensure that it never returns a nil *Location,
	// even if a badly behaved client has changed UTC.
	var utcLoc = Location{name: "UTC"}

	// Local represents the system's local time zone.
	// On Unix systems, Local consults the TZ environment
	// variable to find the time zone to use. No TZ means
	// use the system default /etc/localtime.
	// TZ="" means use UTC.
	// TZ="foo" means use file foo in the system timezone directory.
	var Local *Location = &localLoc

	// localLoc is separate so that initLocal can initialize
	// it even if a client has changed Local.
	var localLoc Location
	var localOnce sync.Once

	func (l Location) get() Location {
	if l == nil {
	return &utcLoc
	}
	if l == &localLoc {
	localOnce.Do(initLocal)
	}
	return l
	}

	// String returns a descriptive name for the time zone information,
	// corresponding to the name argument to LoadLocation or FixedZone.
	func (l *Location) String() string {
	return l.get().name
	}

	var unnamedFixedZones []*Location
	var unnamedFixedZonesOnce sync.Once

	// FixedZone returns a Location that always uses
	// the given zone name and offset (seconds east of UTC).
	func FixedZone(name string, offset int) *Location {
	// Most calls to FixedZone have an unnamed zone with an offset by the hour.
	// Optimize for that case by returning the same *Location for a given hour.
	const hoursBeforeUTC = 12
	const hoursAfterUTC = 14
	hour := offset / 60 / 60
	if name == "" && -hoursBeforeUTC <= hour && hour <= +hoursAfterUTC && hour6060 == offset {
	unnamedFixedZonesOnce.Do(func() {
	unnamedFixedZones = make([]*Location, hoursBeforeUTC+1+hoursAfterUTC)
	for hr := -hoursBeforeUTC; hr <= +hoursAfterUTC; hr++ {
	unnamedFixedZones[hr+hoursBeforeUTC] = fixedZone("", hr6060)
	}
	})
	return unnamedFixedZones[hour+hoursBeforeUTC]
	}
	return fixedZone(name, offset)
	}

	func fixedZone(name string, offset int) *Location {
	l := &Location{
	name: name,
	zone: []zone{{name, offset, false}},
	tx: []zoneTrans{{alpha, 0, false, false}},
	cacheStart: alpha,
	cacheEnd: omega,
	}
	l.cacheZone = &l.zone[0]
	return l
	}

	// lookup returns information about the time zone in use at an
	// instant in time expressed as seconds since January 1, 1970 00:00:00 UTC.
	//
	// The returned information gives the name of the zone (such as "CET"),
	// the start and end times bracketing sec when that zone is in effect,
	// the offset in seconds east of UTC (such as -56060), and whether
	// the daylight savings is being observed at that time.
	func (l *Location) lookup(sec int64) (name string, offset int, start, end int64, isDST bool) {
	l = l.get()

	if len(l.zone) == 0 {
	name = "UTC"
	offset = 0
	start = alpha
	end = omega
	isDST = false
	return
	}

	if zone := l.cacheZone; zone != nil && l.cacheStart <= sec && sec < l.cacheEnd {
	name = zone.name
	offset = zone.offset
	start = l.cacheStart
	end = l.cacheEnd
	isDST = zone.isDST
	return
	}

	if len(l.tx) == 0 \|\| sec < l.tx[0].when {
	zone := &l.zone[l.lookupFirstZone()]
	name = zone.name
	offset = zone.offset
	start = alpha
	if len(l.tx) > 0 {
	end = l.tx[0].when
	} else {
	end = omega
	}
	isDST = zone.isDST
	return
	}

	// Binary search for entry with largest time <= sec.
	// Not using sort.Search to avoid dependencies.
	tx := l.tx
	end = omega
	lo := 0
	hi := len(tx)
	for hi-lo > 1 {
	m := lo + (hi-lo)/2
	lim := tx[m].when
	if sec < lim {
	end = lim
	hi = m
	} else {
	lo = m
	}
	}
	zone := &l.zone[tx[lo].index]
	name = zone.name
	offset = zone.offset
	start = tx[lo].when
	// end = maintained during the search
	isDST = zone.isDST

	// If we're at the end of the known zone transitions,
	// try the extend string.
	if lo == len(tx)-1 && l.extend != "" {
	if ename, eoffset, estart, eend, eisDST, ok := tzset(l.extend, start, sec); ok {
	return ename, eoffset, estart, eend, eisDST
	}
	}

	return
	}

	// lookupFirstZone returns the index of the time zone to use for times
	// before the first transition time, or when there are no transition
	// times.
	//
	// The reference implementation in localtime.c from
	// https://www.iana.org/time-zones/repository/releases/tzcode2013g.tar.gz
	// implements the following algorithm for these cases:
	// 1. If the first zone is unused by the transitions, use it.
	// 2. Otherwise, if there are transition times, and the first
	// transition is to a zone in daylight time, find the first
	// non-daylight-time zone before and closest to the first transition
	// zone.
	// 3. Otherwise, use the first zone that is not daylight time, if
	// there is one.
	// 4. Otherwise, use the first zone.
	func (l *Location) lookupFirstZone() int {
	// Case 1.
	if !l.firstZoneUsed() {
	return 0
	}

	// Case 2.
	if len(l.tx) > 0 && l.zone[l.tx[0].index].isDST {
	for zi := int(l.tx[0].index) - 1; zi >= 0; zi-- {
	if !l.zone[zi].isDST {
	return zi
	}
	}
	}

	// Case 3.
	for zi := range l.zone {
	if !l.zone[zi].isDST {
	return zi
	}
	}

	// Case 4.
	return 0
	}

	// firstZoneUsed reports whether the first zone is used by some
	// transition.
	func (l *Location) firstZoneUsed() bool {
	for _, tx := range l.tx {
	if tx.index == 0 {
	return true
	}
	}
	return false
	}

	// tzset takes a timezone string like the one found in the TZ environment
	// variable, the time of the last time zone transition expressed as seconds
	// since January 1, 1970 00:00:00 UTC, and a time expressed the same way.
	// We call this a tzset string since in C the function tzset reads TZ.
	// The return values are as for lookup, plus ok which reports whether the
	// parse succeeded.
	func tzset(s string, lastTxSec, sec int64) (name string, offset int, start, end int64, isDST, ok bool) {
	var (
	stdName, dstName string
	stdOffset, dstOffset int
	)

	stdName, s, ok = tzsetName(s)
	if ok {
	stdOffset, s, ok = tzsetOffset(s)
	}
	if !ok {
	return "", 0, 0, 0, false, false
	}

	// The numbers in the tzset string are added to local time to get UTC,
	// but our offsets are added to UTC to get local time,
	// so we negate the number we see here.
	stdOffset = -stdOffset

	if len(s) == 0 \|\| s[0] == ',' {
	// No daylight savings time.
	return stdName, stdOffset, lastTxSec, omega, false, true
	}

	dstName, s, ok = tzsetName(s)
	if ok {
	if len(s) == 0 \|\| s[0] == ',' {
	dstOffset = stdOffset + secondsPerHour
	} else {
	dstOffset, s, ok = tzsetOffset(s)
	dstOffset = -dstOffset // as with stdOffset, above
	}
	}
	if !ok {
	return "", 0, 0, 0, false, false
	}

	if len(s) == 0 {
	// Default DST rules per tzcode.
	s = ",M3.2.0,M11.1.0"
	}
	// The TZ definition does not mention ';' here but tzcode accepts it.
	if s[0] != ',' && s[0] != ';' {
	return "", 0, 0, 0, false, false
	}
	s = s[1:]

	var startRule, endRule rule
	startRule, s, ok = tzsetRule(s)
	if !ok \|\| len(s) == 0 \|\| s[0] != ',' {
	return "", 0, 0, 0, false, false
	}
	s = s[1:]
	endRule, s, ok = tzsetRule(s)
	if !ok \|\| len(s) > 0 {
	return "", 0, 0, 0, false, false
	}

	year, _, _, yday := absDate(uint64(sec+unixToInternal+internalToAbsolute), false)

	ysec := int64(yday*secondsPerDay) + sec%secondsPerDay

	// Compute start of year in seconds since Unix epoch.
	d := daysSinceEpoch(year)
	abs := int64(d * secondsPerDay)
	abs += absoluteToInternal + internalToUnix

	startSec := int64(tzruleTime(year, startRule, stdOffset))
	endSec := int64(tzruleTime(year, endRule, dstOffset))
	dstIsDST, stdIsDST := true, false
	// Note: this is a flipping of "DST" and "STD" while retaining the labels
	// This happens in southern hemispheres. The labelling here thus is a little
	// inconsistent with the goal.
	if endSec < startSec {
	startSec, endSec = endSec, startSec
	stdName, dstName = dstName, stdName
	stdOffset, dstOffset = dstOffset, stdOffset
	stdIsDST, dstIsDST = dstIsDST, stdIsDST
	}

	// The start and end values that we return are accurate
	// close to a daylight savings transition, but are otherwise
	// just the start and end of the year. That suffices for
	// the only caller that cares, which is Date.
	if ysec < startSec {
	return stdName, stdOffset, abs, startSec + abs, stdIsDST, true
	} else if ysec >= endSec {
	return stdName, stdOffset, endSec + abs, abs + 365*secondsPerDay, stdIsDST, true
	} else {
	return dstName, dstOffset, startSec + abs, endSec + abs, dstIsDST, true
	}
	}

	// tzsetName returns the timezone name at the start of the tzset string s,
	// and the remainder of s, and reports whether the parsing is OK.
	func tzsetName(s string) (string, string, bool) {
	if len(s) == 0 {
	return "", "", false
	}
	if s[0] != '<' {
	for i, r := range s {
	switch r {
	case '0', '1', '2', '3', '4', '5', '6', '7', '8', '9', ',', '-', '+':
	if i < 3 {
	return "", "", false
	}
	return s[:i], s[i:], true
	}
	}
	if len(s) < 3 {
	return "", "", false
	}
	return s, "", true
	} else {
	for i, r := range s {
	if r == '>' {
	return s[1:i], s[i+1:], true
	}
	}
	return "", "", false
	}
	}

	// tzsetOffset returns the timezone offset at the start of the tzset string s,
	// and the remainder of s, and reports whether the parsing is OK.
	// The timezone offset is returned as a number of seconds.
	func tzsetOffset(s string) (offset int, rest string, ok bool) {
	if len(s) == 0 {
	return 0, "", false
	}
	neg := false
	if s[0] == '+' {
	s = s[1:]
	} else if s[0] == '-' {
	s = s[1:]
	neg = true
	}

	// The tzdata code permits values up to 24 * 7 here,
	// although POSIX does not.
	var hours int
	hours, s, ok = tzsetNum(s, 0, 24*7)
	if !ok {
	return 0, "", false
	}
	off := hours * secondsPerHour
	if len(s) == 0 \|\| s[0] != ':' {
	if neg {
	off = -off
	}
	return off, s, true
	}

	var mins int
	mins, s, ok = tzsetNum(s[1:], 0, 59)
	if !ok {
	return 0, "", false
	}
	off += mins * secondsPerMinute
	if len(s) == 0 \|\| s[0] != ':' {
	if neg {
	off = -off
	}
	return off, s, true
	}

	var secs int
	secs, s, ok = tzsetNum(s[1:], 0, 59)
	if !ok {
	return 0, "", false
	}
	off += secs

	if neg {
	off = -off
	}
	return off, s, true
	}

	// ruleKind is the kinds of rules that can be seen in a tzset string.
	type ruleKind int

	const (
	ruleJulian ruleKind = iota
	ruleDOY
	ruleMonthWeekDay
	)

	// rule is a rule read from a tzset string.
	type rule struct {
	kind ruleKind
	day int
	week int
	mon int
	time int // transition time
	}

	// tzsetRule parses a rule from a tzset string.
	// It returns the rule, and the remainder of the string, and reports success.
	func tzsetRule(s string) (rule, string, bool) {
	var r rule
	if len(s) == 0 {
	return rule{}, "", false
	}
	ok := false
	if s[0] == 'J' {
	var jday int
	jday, s, ok = tzsetNum(s[1:], 1, 365)
	if !ok {
	return rule{}, "", false
	}
	r.kind = ruleJulian
	r.day = jday
	} else if s[0] == 'M' {
	var mon int
	mon, s, ok = tzsetNum(s[1:], 1, 12)
	if !ok \|\| len(s) == 0 \|\| s[0] != '.' {
	return rule{}, "", false

	}
	var week int
	week, s, ok = tzsetNum(s[1:], 1, 5)
	if !ok \|\| len(s) == 0 \|\| s[0] != '.' {
	return rule{}, "", false
	}
	var day int
	day, s, ok = tzsetNum(s[1:], 0, 6)
	if !ok {
	return rule{}, "", false
	}
	r.kind = ruleMonthWeekDay
	r.day = day
	r.week = week
	r.mon = mon
	} else {
	var day int
	day, s, ok = tzsetNum(s, 0, 365)
	if !ok {
	return rule{}, "", false
	}
	r.kind = ruleDOY
	r.day = day
	}

	if len(s) == 0 \|\| s[0] != '/' {
	r.time = 2 * secondsPerHour // 2am is the default
	return r, s, true
	}

	offset, s, ok := tzsetOffset(s[1:])
	if !ok {
	return rule{}, "", false
	}
	r.time = offset

	return r, s, true
	}

	// tzsetNum parses a number from a tzset string.
	// It returns the number, and the remainder of the string, and reports success.
	// The number must be between min and max.
	func tzsetNum(s string, min, max int) (num int, rest string, ok bool) {
	if len(s) == 0 {
	return 0, "", false
	}
	num = 0
	for i, r := range s {
	if r < '0' \|\| r > '9' {
	if i == 0 \|\| num < min {
	return 0, "", false
	}
	return num, s[i:], true
	}
	num *= 10
	num += int(r) - '0'
	if num > max {
	return 0, "", false
	}
	}
	if num < min {
	return 0, "", false
	}
	return num, "", true
	}

	// tzruleTime takes a year, a rule, and a timezone offset,
	// and returns the number of seconds since the start of the year
	// that the rule takes effect.
	func tzruleTime(year int, r rule, off int) int {
	var s int
	switch r.kind {
	case ruleJulian:
	s = (r.day - 1) * secondsPerDay
	if isLeap(year) && r.day >= 60 {
	s += secondsPerDay
	}
	case ruleDOY:
	s = r.day * secondsPerDay
	case ruleMonthWeekDay:
	// Zeller's Congruence.
	m1 := (r.mon+9)%12 + 1
	yy0 := year
	if r.mon <= 2 {
	yy0--
	}
	yy1 := yy0 / 100
	yy2 := yy0 % 100
	dow := ((26m1-2)/10 + 1 + yy2 + yy2/4 + yy1/4 - 2yy1) % 7
	if dow < 0 {
	dow += 7
	}
	// Now dow is the day-of-week of the first day of r.mon.
	// Get the day-of-month of the first "dow" day.
	d := r.day - dow
	if d < 0 {
	d += 7
	}
	for i := 1; i < r.week; i++ {
	if d+7 >= daysIn(Month(r.mon), year) {
	break
	}
	d += 7
	}
	d += int(daysBefore[r.mon-1])
	if isLeap(year) && r.mon > 2 {
	d++
	}
	s = d * secondsPerDay
	}

	return s + r.time - off
	}

	// lookupName returns information about the time zone with
	// the given name (such as "EST") at the given pseudo-Unix time
	// (what the given time of day would be in UTC).
	func (l *Location) lookupName(name string, unix int64) (offset int, ok bool) {
	l = l.get()

	// First try for a zone with the right name that was actually
	// in effect at the given time. (In Sydney, Australia, both standard
	// and daylight-savings time are abbreviated "EST". Using the
	// offset helps us pick the right one for the given time.
	// It's not perfect: during the backward transition we might pick
	// either one.)
	for i := range l.zone {
	zone := &l.zone[i]
	if zone.name == name {
	nam, offset, _, _, _ := l.lookup(unix - int64(zone.offset))
	if nam == zone.name {
	return offset, true
	}
	}
	}

	// Otherwise fall back to an ordinary name match.
	for i := range l.zone {
	zone := &l.zone[i]
	if zone.name == name {
	return zone.offset, true
	}
	}

	// Otherwise, give up.
	return
	}

	// NOTE(rsc): Eventually we will need to accept the POSIX TZ environment
	// syntax too, but I don't feel like implementing it today.

	var errLocation = errors.New("time: invalid location name")

	var zoneinfo *string
	var zoneinfoOnce sync.Once

	// LoadLocation returns the Location with the given name.
	//
	// If the name is "" or "UTC", LoadLocation returns UTC.
	// If the name is "Local", LoadLocation returns Local.
	//
	// Otherwise, the name is taken to be a location name corresponding to a file
	// in the IANA Time Zone database, such as "America/New_York".
	//
	// LoadLocation looks for the IANA Time Zone database in the following
	// locations in order:
	//
	// - the directory or uncompressed zip file named by the ZONEINFO environment variable
	// - on a Unix system, the system standard installation location
	// - $GOROOT/lib/time/zoneinfo.zip
	// - the time/tzdata package, if it was imported
	func LoadLocation(name string) (*Location, error) {
	if name == "" \|\| name == "UTC" {
	return UTC, nil
	}
	if name == "Local" {
	return Local, nil
	}
	if containsDotDot(name) \|\| name[0] == '/' \|\| name[0] == '\\' {
	// No valid IANA Time Zone name contains a single dot,
	// much less dot dot. Likewise, none begin with a slash.
	return nil, errLocation
	}
	zoneinfoOnce.Do(func() {
	env, _ := syscall.Getenv("ZONEINFO")
	zoneinfo = &env
	})
	var firstErr error
	if *zoneinfo != "" {
	if zoneData, err := loadTzinfoFromDirOrZip(*zoneinfo, name); err == nil {
	if z, err := LoadLocationFromTZData(name, zoneData); err == nil {
	return z, nil
	}
	firstErr = err
	} else if err != syscall.ENOENT {
	firstErr = err
	}
	}
	if z, err := loadLocation(name, platformZoneSources); err == nil {
	return z, nil
	} else if firstErr == nil {
	firstErr = err
	}
	return nil, firstErr
	}

	// containsDotDot reports whether s contains "..".
	func containsDotDot(s string) bool {
	if len(s) < 2 {
	return false
	}
	for i := 0; i < len(s)-1; i++ {
	if s[i] == '.' && s[i+1] == '.' {
	return true
	}
	}
	return false
	}