time: fix Time.Add

R=rsc, r
CC=golang-dev
https://golang.org/cl/5448121

src/pkg/time/time.go

@@ -548,7 +548,7 @@ func (d Duration) Hours() float64 {
 func (t Time) Add(d Duration) Time {
 	t.sec += int64(d / 1e9)
 	t.nsec += int32(d % 1e9)
-	if t.nsec > 1e9 {
+	if t.nsec >= 1e9 {
 		t.sec++
 		t.nsec -= 1e9
 	} else if t.nsec < 0 {

src/pkg/time/time_test.go

@@ -655,6 +655,17 @@ func TestDaysIn(t *testing.T) {
 	}
 }
 
+func TestAddToExactSecond(t *testing.T) {
+	// Add an amount to the current time to round it up to the next exact second.
+	// This test checks that the nsec field still lies within the range [0, 999999999].
+	t1 := Now()
+	t2 := t1.Add(Second - Duration(t1.Nanosecond()))
+	sec := (t1.Second() + 1) % 60
+	if t2.Second() != sec || t2.Nanosecond() != 0 {
+		t.Errorf("sec = %d, nsec = %d, want sec = %d, nsec = 0", t2.Second(), t2.Nanosecond(), sec)
+	}
+}
+
 func BenchmarkNow(b *testing.B) {
 	for i := 0; i < b.N; i++ {
 		Now()