add Until

clock.go

@@ -19,6 +19,7 @@ type Clock interface {
 	AfterFunc(d time.Duration, f func()) *Timer
 	Now() time.Time
 	Since(t time.Time) time.Duration
+	Until(t time.Time) time.Duration
 	Sleep(d time.Duration)
 	Tick(d time.Duration) <-chan time.Time
 	Ticker(d time.Duration) *Ticker
@@ -43,6 +44,8 @@ func (c *clock) Now() time.Time { return time.Now() }
 
 func (c *clock) Since(t time.Time) time.Duration { return time.Since(t) }
 
+func (c *clock) Until(t time.Time) time.Duration { return time.Until(t) }
+
 func (c *clock) Sleep(d time.Duration) { time.Sleep(d) }
 
 func (c *clock) Tick(d time.Duration) <-chan time.Time { return time.Tick(d) }
@@ -164,11 +167,16 @@ func (m *Mock) Now() time.Time {
 	return m.now
 }
 
-// Since returns time since the mock clock's wall time.
+// Since returns time since `t` using the mock clock's wall time.
 func (m *Mock) Since(t time.Time) time.Duration {
 	return m.Now().Sub(t)
 }
 
+// Until returns time until `t` using the mock clock's wall time.
+func (m *Mock) Until(t time.Time) time.Duration {
+	return t.Sub(m.Now())
+}
+
 // Sleep pauses the goroutine for the given duration on the mock clock.
 // The clock must be moved forward in a separate goroutine.
 func (m *Mock) Sleep(d time.Duration) {

clock_test.go

@@ -295,6 +295,19 @@ func TestMock_Since(t *testing.T) {
 	}
 }
 
+func TestMock_Until(t *testing.T) {
+	clock := NewMock()
+
+	end := clock.Now().Add(500 * time.Second)
+	if dur := clock.Until(end); dur.Seconds() != 500 {
+		t.Fatalf("expected 500s duration between `clock` and `end`, actually: %v", dur.Seconds())
+	}
+	clock.Add(100 * time.Second)
+	if dur := clock.Until(end); dur.Seconds() != 400 {
+		t.Fatalf("expected 400s duration between `clock` and `end`, actually: %v", dur.Seconds())
+	}
+}
+
 // Ensure that the mock can sleep for the correct time.
 func TestMock_Sleep(t *testing.T) {
 	var ok int32