1 files changed, 80 insertions, 7 deletions
diff --git a/src/pkg/time/zoneinfo.go b/src/pkg/time/zoneinfo.go
index 1c6186258..c8e53a27c 100644
--- a/src/pkg/time/zoneinfo.go
+++ b/src/pkg/time/zoneinfo.go
@@ -45,6 +45,13 @@ type zoneTrans struct {
 	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
 
@@ -83,9 +90,9 @@ func FixedZone(name string, offset int) *Location {
 	l := &Location{
 		name:       name,
 		zone:       []zone{{name, offset, false}},
-		tx:         []zoneTrans{{-1 << 63, 0, false, false}},
-		cacheStart: -1 << 63,
-		cacheEnd:   1<<63 - 1,
+		tx:         []zoneTrans{{alpha, 0, false, false}},
+		cacheStart: alpha,
+		cacheEnd:   omega,
 	}
 	l.cacheZone = &l.zone[0]
 	return l
@@ -101,12 +108,12 @@ func FixedZone(name string, offset int) *Location {
 func (l *Location) lookup(sec int64) (name string, offset int, isDST bool, start, end int64) {
 	l = l.get()
 
-	if len(l.tx) == 0 {
+	if len(l.zone) == 0 {
 		name = "UTC"
 		offset = 0
 		isDST = false
-		start = -1 << 63
-		end = 1<<63 - 1
+		start = alpha
+		end = omega
 		return
 	}
 
@@ -119,10 +126,24 @@ func (l *Location) lookup(sec int64) (name string, offset int, isDST bool, start
 		return
 	}
 
+	if len(l.tx) == 0 || sec < l.tx[0].when {
+		zone := &l.zone[l.lookupFirstZone()]
+		name = zone.name
+		offset = zone.offset
+		isDST = zone.isDST
+		start = alpha
+		if len(l.tx) > 0 {
+			end = l.tx[0].when
+		} else {
+			end = omega
+		}
+		return
+	}
+
 	// Binary search for entry with largest time <= sec.
 	// Not using sort.Search to avoid dependencies.
 	tx := l.tx
-	end = 1<<63 - 1
+	end = omega
 	lo := 0
 	hi := len(tx)
 	for hi-lo > 1 {
@@ -144,6 +165,58 @@ func (l *Location) lookup(sec int64) (name string, offset int, isDST bool, start
 	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
+// http://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 returns 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
+}
+
 // 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).