diff --git a/meter.go b/meter.go
index 0389ab0..9adce5a 100644
--- a/meter.go
+++ b/meter.go
@@ -190,10 +190,13 @@ func (m *StandardMeter) Snapshot() Meter {
 func (m *StandardMeter) updateSnapshot() {
 	// should run with write lock held on m.lock
 	snapshot := m.snapshot
-	snapshot.rate1 = m.a1.Rate()
-	snapshot.rate5 = m.a5.Rate()
-	snapshot.rate15 = m.a15.Rate()
-	snapshot.rateMean = float64(snapshot.count) / time.Since(m.startTime).Seconds()
+	elapsed := time.Since(m.startTime).Seconds()
+	if elapsed >= 0 {
+		snapshot.rate1 = m.a1.Rate()
+		snapshot.rate5 = m.a5.Rate()
+		snapshot.rate15 = m.a15.Rate()
+		snapshot.rateMean = float64(snapshot.count) / elapsed
+	}
 }
 
 func (m *StandardMeter) tick() {
diff --git a/timer.go b/timer.go
index 17db8f8..3eb1225 100644
--- a/timer.go
+++ b/timer.go
@@ -215,18 +215,18 @@ func (t *StandardTimer) Time(f func()) {
 
 // Record the duration of an event.
 func (t *StandardTimer) Update(d time.Duration) {
-	t.mutex.Lock()
-	defer t.mutex.Unlock()
-	t.histogram.Update(int64(d))
-	t.meter.Mark(1)
+	// Partial workaround for detecting a clock change
+	if d >= 0 {
+		t.mutex.Lock()
+		defer t.mutex.Unlock()
+		t.histogram.Update(int64(d))
+		t.meter.Mark(1)
+	}
 }
 
 // Record the duration of an event that started at a time and ends now.
 func (t *StandardTimer) UpdateSince(ts time.Time) {
-	t.mutex.Lock()
-	defer t.mutex.Unlock()
-	t.histogram.Update(int64(time.Since(ts)))
-	t.meter.Mark(1)
+	t.Update(time.Since(ts))
 }
 
 // Variance returns the variance of the values in the sample.
diff --git a/timer_test.go b/timer_test.go
index 313d691..f908da3 100644
--- a/timer_test.go
+++ b/timer_test.go
@@ -27,6 +27,7 @@ func TestTimerExtremes(t *testing.T) {
 	tm := NewTimer()
 	tm.Update(math.MaxInt64)
 	tm.Update(0)
+	tm.Update(math.MinInt64)
 	if stdDev := tm.StdDev(); 4.611686018427388e+18 != stdDev {
 		t.Errorf("tm.StdDev(): 4.611686018427388e+18 != %v\n", stdDev)
 	}