model: add Duration unit constants and conversion methods

model/time.go

@@ -164,6 +164,34 @@ func (t *Time) UnmarshalJSON(b []byte) error {
 // This type should not propagate beyond the scope of input/output processing.
 type Duration time.Duration
 
+// Common durations. Day and Week are included because ParseDuration supports
+// those units (unlike the standard time package).
+//
+// To count the number of units in a Duration, divide:
+//
+//	second := model.Second
+//	fmt.Print(int64(second/model.Millisecond)) // prints 1000
+//
+// To convert an integer number of units to a Duration, multiply:
+//
+//	seconds := 10
+//	fmt.Print(model.Duration(seconds)*model.Second) // prints 10s
+const (
+	Nanosecond  Duration = 1
+	Microsecond          = 1000 * Nanosecond
+	Millisecond          = 1000 * Microsecond
+	Second               = 1000 * Millisecond
+	Minute               = 60 * Second
+	Hour                 = 60 * Minute
+	Day                  = 24 * Hour
+	Week                 = 7 * Day
+)
+
+// Milliseconds returns the duration as an integer millisecond count.
+// Prometheus stores timestamps in milliseconds; time.Duration already
+// covers the other units.
+func (d Duration) Milliseconds() int64 { return time.Duration(d).Milliseconds() }
+
 // Set implements pflag/flag.Value.
 func (d *Duration) Set(s string) error {
 	var err error

model/time_test.go

@@ -67,6 +67,29 @@ func TestDuration(t *testing.T) {
 	require.Equalf(t, delta, duration, "Expected %s to be equal to %s", delta, duration)
 }
 
+func TestDurationConstants(t *testing.T) {
+	// require.Equal(expected, actual) — expected is the reference time.Duration cast.
+	require.Equal(t, Nanosecond, Duration(time.Nanosecond))
+	require.Equal(t, Microsecond, Duration(time.Microsecond))
+	require.Equal(t, Millisecond, Duration(time.Millisecond))
+	require.Equal(t, Second, Duration(time.Second))
+	require.Equal(t, Minute, Duration(time.Minute))
+	require.Equal(t, Hour, Duration(time.Hour))
+	require.Equal(t, Day, Duration(24*time.Hour))
+	require.Equal(t, Week, Duration(7*24*time.Hour))
+
+	// Typical usage from the issue: multiply integers by model constants.
+	require.Equal(t, 15*Day, Duration(15*24*time.Hour))
+	require.Equal(t, 5000*Millisecond, Duration(5000*time.Millisecond))
+}
+
+func TestDurationMilliseconds(t *testing.T) {
+	d := 2*Hour + 30*Minute + 15*Second + 250*Millisecond
+	require.Equal(t, time.Duration(d).Milliseconds(), d.Milliseconds())
+	require.Equal(t, int64(2*60*60*1000+30*60*1000+15*1000+250), d.Milliseconds())
+	require.Equal(t, int64(-1500), (-1500 * Millisecond).Milliseconds())
+}
+
 func TestParseDuration(t *testing.T) {
 	type testCase struct {
 		in              string