fix: preserve an explicit year zero in TimestampFlag

flag_test.go

@@ -2910,6 +2910,27 @@ func TestTimestampFlagApply_YearlessLayoutJanuaryFirst(t *testing.T) {
 	}
 }
 
+func TestTimestampFlagApply_ExplicitYearZero(t *testing.T) {
+	for _, tc := range []struct{ layout, value string }{
+		{time.RFC3339, "0000-02-29T12:34:56Z"},
+		{time.DateOnly, "0000-01-01"},
+		{"2006 15:04", "0000 12:34"},
+	} {
+		t.Run(tc.layout, func(t *testing.T) {
+			want, err := time.Parse(tc.layout, tc.value)
+			require.NoError(t, err)
+			cmd := &Command{
+				Flags: []Flag{&TimestampFlag{Name: "time", Config: TimestampConfig{Layouts: []string{tc.layout}}}},
+				Action: func(_ context.Context, cmd *Command) error {
+					assert.Equal(t, want, cmd.Timestamp("time"))
+					return nil
+				},
+			}
+			assert.NoError(t, cmd.Run(buildTestContext(t), []string{"app", "--time", tc.value}))
+		})
+	}
+}
+
 func TestTimestampFlagApplyValue(t *testing.T) {
 	expectedResult, _ := time.Parse(time.RFC3339, "2006-01-02T15:04:05Z")
 	fl := TimestampFlag{Name: "time", Aliases: []string{"t"}, Config: TimestampConfig{Layouts: []string{time.RFC3339}}, Value: expectedResult}

flag_timestamp.go

@@ -94,7 +94,9 @@ func (t *timestampValue) Set(value string) error {
 	// The date is compared in the timestamp's own location: truncating the
 	// instant to a day works in UTC and misses January 1 of year 0 for part
 	// of the day whenever Timezone is not UTC.
-	if !layoutHasDate(matchedLayout) && timestamp.Year() == 0 && timestamp.YearDay() == 1 {
+	hasYear, hasDate := layoutFields(matchedLayout)
+	yearMissing := timestamp.Year() == 0 && !hasYear
+	if !hasDate && yearMissing && timestamp.YearDay() == 1 {
 		timestamp = time.Date(
 			n.Year(),
 			n.Month(),
@@ -105,7 +107,7 @@ func (t *timestampValue) Set(value string) error {
 			timestamp.Nanosecond(),
 			timestamp.Location(),
 		)
-	} else if timestamp.Year() == 0 {
+	} else if yearMissing {
 		timestamp = time.Date(
 			n.Year(),
 			timestamp.Month(),
@@ -125,12 +127,15 @@ func (t *timestampValue) Set(value string) error {
 	return nil
 }
 
-// layoutHasDate reports whether layout carries month/day components, probed by
-// round-tripping a reference date that is not January 1.
-func layoutHasDate(layout string) bool {
+// layoutFields reports whether layout carries a year and month/day components,
+// probed by round-tripping a reference date that is not January 1 of year 0.
+func layoutFields(layout string) (hasYear, hasDate bool) {
 	ref := time.Date(2, time.March, 4, 5, 6, 7, 0, time.UTC)
-	p, err := time.Parse(layout, ref.Format(layout))
-	return err == nil && (p.Month() != time.January || p.Day() != 1)
+	if p, err := time.Parse(layout, ref.Format(layout)); err == nil {
+		hasYear = p.Year() != 0
+		hasDate = p.Month() != time.January || p.Day() != 1
+	}
+	return hasYear, hasDate
 }
 
 // String returns a readable representation of this value (for usage defaults)