diff --git a/src/time/time.go b/src/time/time.go
index 144f2fe73d..4241a6241b 100644
--- a/src/time/time.go
+++ b/src/time/time.go
@@ -908,13 +908,27 @@ func (t Time) Sub(u Time) Duration {
 // Since returns the time elapsed since t.
 // It is shorthand for time.Now().Sub(t).
 func Since(t Time) Duration {
-	return Now().Sub(t)
+	var now Time
+	if t.wall&hasMonotonic != 0 {
+		// Common case optimization: if t has monotomic time, then Sub will use only it.
+		now = Time{hasMonotonic, runtimeNano() - startNano, nil}
+	} else {
+		now = Now()
+	}
+	return now.Sub(t)
 }
 
 // Until returns the duration until t.
 // It is shorthand for t.Sub(time.Now()).
 func Until(t Time) Duration {
-	return t.Sub(Now())
+	var now Time
+	if t.wall&hasMonotonic != 0 {
+		// Common case optimization: if t has monotomic time, then Sub will use only it.
+		now = Time{hasMonotonic, runtimeNano() - startNano, nil}
+	} else {
+		now = Now()
+	}
+	return t.Sub(now)
 }
 
 // AddDate returns the time corresponding to adding the