diff --git a/clock/clock.go b/clock/clock.go
index e8f0b36..52f34cd 100644
--- a/clock/clock.go
+++ b/clock/clock.go
@@ -22,11 +22,11 @@ type Clock struct {
 	wall uint64
 }
 
-func abs[Int int | uint64](wall Int) uint64 {
+func abs[Int int | float64 | uint64](wall Int) uint64 {
 	for wall < 0 {
 		wall += secondsPerDay
 	}
-	return uint64(wall % secondsPerDay)
+	return uint64(int64(wall) % secondsPerDay)
 }
 
 func New(hour, min, sec int) Clock {
@@ -134,7 +134,7 @@ func (c Clock) Compare(u Clock) int {
 }
 
 func (c Clock) Add(d time.Duration) Clock {
-	return Clock{abs(c.wall + uint64(d.Seconds()))}
+	return Clock{abs(float64(c.wall) + d.Seconds())}
 }
 
 func (c Clock) Sub(u Clock) time.Duration {
diff --git a/clock/clock_test.go b/clock/clock_test.go
index 028e504..29e6893 100644
--- a/clock/clock_test.go
+++ b/clock/clock_test.go
@@ -74,6 +74,22 @@ func TestSeconds(t *testing.T) {
 	}
 }
 
+func TestAdd(t *testing.T) {
+	for i, tc := range []struct {
+		c Clock
+		d time.Duration
+		u Clock
+	}{
+		{Clock{}, 0, Clock{}},
+		{Clock{}, time.Second, New(0, 0, 1)},
+		{New(0, 0, 1), -time.Second, Clock{}},
+	} {
+		if got := tc.c.Add(tc.d); got != tc.u {
+			t.Errorf("#%d: got %s; want %s", i, got, tc.u)
+		}
+	}
+}
+
 func TestSub(t *testing.T) {
 	for i, tc := range []struct {
 		c Clock
@@ -88,7 +104,7 @@ func TestSub(t *testing.T) {
 		{New(12, 0, 0), New(12, 30, 0), -30 * time.Minute},
 	} {
 		if got := tc.c.Sub(tc.u); got != tc.d {
-			t.Errorf("#%d: Sub(%v, %v): got %v; want %v", i, tc.c, tc.u, got, tc.d)
+			t.Errorf("#%d: got %v; want %v", i, got, tc.d)
 		}
 	}
 }