time: use extended time format past end of zone transitions

This gives us better expected information for daylight savings time
transitions in year 2038 and beyond.

Fixes #36654

Change-Id: I5a39aed3c40b184e1d7bb7d6ce3aff5307c4c146
Reviewed-on: https://go-review.googlesource.com/c/go/+/215539
Run-TryBot: Ian Lance Taylor <[email protected]>
TryBot-Result: Gobot Gobot <[email protected]>
Reviewed-by: Brad Fitzpatrick <[email protected]>

src/time/export_test.go

@@ -36,12 +36,43 @@ var (
 	ReadFile               = readFile
 	LoadTzinfo             = loadTzinfo
 	NextStdChunk           = nextStdChunk
+	Tzset                  = tzset
+	TzsetName              = tzsetName
+	TzsetOffset            = tzsetOffset
 )
 
 func LoadFromEmbeddedTZData(zone string) (string, error) {
 	return loadFromEmbeddedTZData(zone)
 }
 
+type RuleKind int
+
+const (
+	RuleJulian       = RuleKind(ruleJulian)
+	RuleDOY          = RuleKind(ruleDOY)
+	RuleMonthWeekDay = RuleKind(ruleMonthWeekDay)
+)
+
+type Rule struct {
+	Kind RuleKind
+	Day  int
+	Week int
+	Mon  int
+	Time int
+}
+
+func TzsetRule(s string) (Rule, string, bool) {
+	r, rs, ok := tzsetRule(s)
+	rr := Rule{
+		Kind: RuleKind(r.kind),
+		Day:  r.day,
+		Week: r.week,
+		Mon:  r.mon,
+		Time: r.time,
+	}
+	return rr, rs, ok
+}
+
 // StdChunkNames maps from nextStdChunk results to the matched strings.
 var StdChunkNames = map[int]string{
 	0:                               "",

src/time/time.go

@@ -1019,6 +1019,34 @@ func daysIn(m Month, year int) int {
 	return int(daysBefore[m] - daysBefore[m-1])
 }
 
+// daysSinceEpoch takes a year and returns the number of days from
+// the absolute epoch to the start of that year.
+// This is basically (year - zeroYear) * 365, but accounting for leap days.
+func daysSinceEpoch(year int) uint64 {
+	y := uint64(int64(year) - absoluteZeroYear)
+
+	// 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
+
+	return d
+}
+
 // Provided by package runtime.
 func now() (sec int64, nsec int32, mono int64)
 
@@ -1327,28 +1355,8 @@ func Date(year int, month Month, day, hour, min, sec, nsec int, loc *Location) T
 	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
+	d := daysSinceEpoch(year)
 
 	// Add in days before this month.
 	d += uint64(daysBefore[month-1])

src/time/time_test.go

@@ -66,6 +66,13 @@ var nanoutctests = []TimeTest{
 var localtests = []TimeTest{
 	{0, parsedTime{1969, December, 31, 16, 0, 0, 0, Wednesday, -8 * 60 * 60, "PST"}},
 	{1221681866, parsedTime{2008, September, 17, 13, 4, 26, 0, Wednesday, -7 * 60 * 60, "PDT"}},
+	{2159200800, parsedTime{2038, June, 3, 11, 0, 0, 0, Thursday, -7 * 60 * 60, "PDT"}},
+	{2152173599, parsedTime{2038, March, 14, 1, 59, 59, 0, Sunday, -8 * 60 * 60, "PST"}},
+	{2152173600, parsedTime{2038, March, 14, 3, 0, 0, 0, Sunday, -7 * 60 * 60, "PDT"}},
+	{2152173601, parsedTime{2038, March, 14, 3, 0, 1, 0, Sunday, -7 * 60 * 60, "PDT"}},
+	{2172733199, parsedTime{2038, November, 7, 1, 59, 59, 0, Sunday, -7 * 60 * 60, "PDT"}},
+	{2172733200, parsedTime{2038, November, 7, 1, 0, 0, 0, Sunday, -8 * 60 * 60, "PST"}},
+	{2172733201, parsedTime{2038, November, 7, 1, 0, 1, 0, Sunday, -8 * 60 * 60, "PST"}},
 }
 
 var nanolocaltests = []TimeTest{