Set json encoding precision

parser_test.go

@@ -642,3 +642,49 @@ func benchmarkSigning(b *testing.B, method jwt.SigningMethod, key interface{}) {
 		}
 	})
 }
+
+func TestNewNumericDateMarshalJSON(t *testing.T) {
+	// Do not run this test in parallel because it's changing
+	// global state.
+	oldPrecision := jwt.TimePrecision
+	t.Cleanup(func() {
+		jwt.TimePrecision = oldPrecision
+	})
+
+	tt := []struct {
+		in        time.Time
+		want      string
+		precision time.Duration
+	}{
+		{time.Unix(5243700879, 0), "5243700879", time.Second},
+		{time.Unix(5243700879, 0), "5243700879.000001", time.Millisecond},
+		{time.Unix(5243700879, 0), "5243700879.000001", time.Microsecond},
+		{time.Unix(5243700879, 0), "5243700879.000001", time.Nanosecond},
+		//
+		{time.Unix(4239425898, 0), "4239425898", time.Second},
+		{time.Unix(4239425898, 0), "4239425898", time.Millisecond},
+		{time.Unix(4239425898, 0), "4239425898", time.Microsecond},
+		{time.Unix(4239425898, 0), "4239425898", time.Nanosecond},
+		//
+		{time.Unix(0, 1644285000210402000), "1644285000", time.Second},
+		{time.Unix(0, 1644285000210402000), "1644285000.2099998", time.Millisecond},
+		{time.Unix(0, 1644285000210402000), "1644285000.210402", time.Microsecond},
+		{time.Unix(0, 1644285000210402000), "1644285000.210402", time.Nanosecond},
+		//
+		{time.Unix(0, 1644285315063096000), "1644285315", time.Second},
+		{time.Unix(0, 1644285315063096000), "1644285315.063", time.Millisecond},
+		{time.Unix(0, 1644285315063096000), "1644285315.063096", time.Microsecond},
+		{time.Unix(0, 1644285315063096000), "1644285315.063096", time.Nanosecond},
+	}
+
+	for i, tc := range tt {
+		jwt.TimePrecision = tc.precision
+		by, err := jwt.NewNumericDate(tc.in).MarshalJSON()
+		if err != nil {
+			t.Fatal(err)
+		}
+		if got := string(by); got != tc.want {
+			t.Fatalf("[%d]: failed encoding: got %q want %q", i, got, tc.want)
+		}
+	}
+}

types.go

@@ -49,9 +49,13 @@ func newNumericDateFromSeconds(f float64) *NumericDate {
 // MarshalJSON is an implementation of the json.RawMessage interface and serializes the UNIX epoch
 // represented in NumericDate to a byte array, using the precision specified in TimePrecision.
 func (date NumericDate) MarshalJSON() (b []byte, err error) {
+	prec := -1
+	if TimePrecision == time.Second {
+		prec = 0
+	}
 	f := float64(date.Truncate(TimePrecision).UnixNano()) / float64(time.Second)
 
-	return []byte(strconv.FormatFloat(f, 'f', -1, 64)), nil
+	return []byte(strconv.FormatFloat(f, 'f', prec, 64)), nil
 }
 
 // UnmarshalJSON is an implementation of the json.RawMessage interface and deserializses a