time: new AddDate method

R=rsc
CC=golang-dev
https://golang.org/cl/5465044

src/pkg/time/time.go

@@ -564,6 +564,20 @@ func (t Time) Sub(u Time) Duration {
 	return Duration(t.sec-u.sec)*Second + Duration(t.nsec-u.nsec)
 }
 
+// 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

src/pkg/time/time_test.go

@@ -634,6 +634,32 @@ func TestDate(t *testing.T) {
 	}
 }
 
+// Several ways of getting from
+// Fri Nov 18 7:56:35 PST 2011
+// to
+// Thu Mar 19 7:56:35 PST 2016
+var addDateTests = []struct {
+	years, months, days int
+}{
+	{4, 4, 1},
+	{3, 16, 1},
+	{3, 15, 30},
+	{5, -6, -18 - 30 - 12},
+}
+
+func TestAddDate(t *testing.T) {
+	t0 := Date(2011, 11, 18, 7, 56, 35, 0, UTC)
+	t1 := Date(2016, 3, 19, 7, 56, 35, 0, UTC)
+	for _, at := range addDateTests {
+		time := t0.AddDate(at.years, at.months, at.days)
+		if !time.Equal(t1) {
+			t.Errorf("AddDate(%d, %d, %d) = %v, want %v",
+				at.years, at.months, at.days,
+				time, t1)
+		}
+	}
+}
+
 var daysInTests = []struct {
 	year, month, di int
 }{