expression: keep month and day when a year/month interval underflows to year 0

pkg/expression/builtin_time.go

@@ -3204,7 +3204,19 @@ func (du *baseDateArithmetical) addDate(ctx EvalContext, date types.Time, year,
 	// fix https://github.com/pingcap/tidb/issues/11329
 	if goTime.Year() == 0 {
 		hour, minute, second := goTime.Clock()
-		date.SetCoreTime(types.FromDate(0, 0, 0, hour, minute, second, goTime.Nanosecond()/1000))
+		// Year/month intervals do calendar arithmetic, so when they drive the year down
+		// to 0 MySQL keeps the month and day, e.g.
+		// DATE_SUB('1000-01-01', INTERVAL 1000 YEAR) -> 0000-01-01 (issue #59789).
+		// Day/time intervals instead underflow past the minimum datetime and yield a
+		// zero date with the wrapped time, e.g.
+		// DATE_ADD('0001-01-01 00:00:00', INTERVAL -2 HOUR) -> 0000-00-00 22:00:00 (issue #11329).
+		// A nonzero year or month component is what distinguishes the calendar case;
+		// intervals without one (including a zero interval) keep the zero-date normalization.
+		if (year != 0 || month != 0) && day == 0 && nano == 0 {
+			date.SetCoreTime(types.FromDate(0, int(goTime.Month()), goTime.Day(), hour, minute, second, goTime.Nanosecond()/1000))
+		} else {
+			date.SetCoreTime(types.FromDate(0, 0, 0, hour, minute, second, goTime.Nanosecond()/1000))
+		}
 		return date, false, nil
 	}
 

pkg/expression/builtin_time_test.go

@@ -2396,6 +2396,9 @@ func TestDateArithFuncs(t *testing.T) {
 		{"2001-02-28", -1, "2000-02-28"},
 		{"2004-02-29", 1, "2005-02-28"},
 		{"2005-02-28", -1, "2004-02-28"},
+		// A YEAR interval that drives the year down to 0 keeps the month and day,
+		// matching MySQL instead of zeroing them out. See issue #59789.
+		{"1000-01-01", -1000, "0000-01-01"},
 	}
 
 	for _, test := range testYears {