time: restore support for large negative days in Date

CL 586257 converted days to uint32 which is usually fine but fails for
negative days close to cycle boundaries.

Fixes #68718

Change-Id: I8dc5b8fe0c7c1921beb204da1913b9a1ab39280d
Reviewed-on: https://go-review.googlesource.com/c/go/+/602815
Reviewed-by: Rob Pike <r@golang.org>
Auto-Submit: Ian Lance Taylor <iant@google.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

src/time/time.go

@@ -591,6 +591,7 @@ type absJanFeb int
 
 // dateToAbsDays takes a standard year/month/day and returns the
 // number of days from the absolute epoch to that day.
+// The days argument can be out of range and in particular can be negative.
 func dateToAbsDays(year int64, month Month, day int) absDays {
 	// See “Computations on Times” comment above.
 	amonth := uint32(month)
@@ -626,7 +627,7 @@ func dateToAbsDays(year int64, month Month, day int) absDays {
 	cday := 1461 * cyear / 4
 	centurydays := 146097 * century / 4
 
-	return absDays(centurydays + uint64(cday+ayday+uint32(day)-1))
+	return absDays(centurydays + uint64(int64(cday+ayday)+int64(day)-1))
 }
 
 // days converts absolute seconds to absolute days.

src/time/time_test.go

@@ -666,6 +666,9 @@ var dateTests = []struct {
 	{2012, 1, -43, 7, 56, 35, 0, Local, 1321631795},                 // Jan -52 7:56:35 2012
 	{2012, int(January - 2), 18, 7, 56, 35, 0, Local, 1321631795},   // (Jan-2) 18 7:56:35 2012
 	{2010, int(December + 11), 18, 7, 56, 35, 0, Local, 1321631795}, // (Dec+11) 18 7:56:35 2010
+	{1970, 1, 15297, 7, 56, 35, 0, Local, 1321631795},               // large number of days
+
+	{1970, 1, -25508, 0, 0, 0, 0, Local, -2203948800}, // negative Unix time
 }
 
 func TestDate(t *testing.T) {
@@ -704,6 +707,13 @@ func TestAddDate(t *testing.T) {
 				time, t1)
 		}
 	}
+
+	t2 := Date(1899, 12, 31, 0, 0, 0, 0, UTC)
+	days := t2.Unix() / (24 * 60 * 60)
+	t3 := Unix(0, 0).AddDate(0, 0, int(days))
+	if !t2.Equal(t3) {
+		t.Errorf("Adddate(0, 0, %d) = %v, want %v", days, t3, t2)
+	}
 }
 
 var daysInTests = []struct {