Fix incorrect comment in rtptime.go.

Also use Duration.Abs.

rtptime/rtptime.go

@@ -10,10 +10,9 @@ import (
 var epoch = time.Now()
 
 // FromDuration converts a time.Duration into units of 1/hz.
-// Negative values are clamped to zero.
 func FromDuration(d time.Duration, hz uint32) int64 {
 	if d < 0 {
-		return -FromDuration(-d, hz)
+		return -FromDuration(d.Abs(), hz)
 	}
 	hi, lo := bits.Mul64(uint64(d), uint64(hz))
 	q, _ := bits.Div64(hi, lo, uint64(time.Second))

rtptime/rtptime_test.go

@@ -82,10 +82,7 @@ func TestNTP(t *testing.T) {
 	now2 := NTPToTime(ntp)
 	ntp2 := TimeToNTP(now2)
 
-	diff1 := now2.Sub(now)
-	if diff1 < 0 {
-		diff1 = -diff1
-	}
+	diff1 := now2.Sub(now).Abs()
 	if diff1 > time.Nanosecond {
 		t.Errorf("Expected %v, got %v (diff=%v)",
 			now, now2, diff1)