diff --git a/core/collection/rollingwindow.go b/core/collection/rollingwindow.go
index b6fbe408..51b0d87c 100644
--- a/core/collection/rollingwindow.go
+++ b/core/collection/rollingwindow.go
@@ -96,7 +96,7 @@ func (rw *RollingWindow) updateOffset() {
 	}
 
 	rw.offset = (offset + span) % rw.size
-	rw.lastTime = timex.Now()
+	rw.lastTime = time.Duration(int(rw.lastTime) + int(rw.interval)*span)
 }
 
 type Bucket struct {
diff --git a/core/collection/rollingwindow_test.go b/core/collection/rollingwindow_test.go
index 9bb88845..b4e05b14 100644
--- a/core/collection/rollingwindow_test.go
+++ b/core/collection/rollingwindow_test.go
@@ -45,6 +45,31 @@ func TestRollingWindowAdd(t *testing.T) {
 	assert.Equal(t, []float64{5, 15, 7}, listBuckets())
 }
 
+func TestRollingWindowAdd2(t *testing.T) {
+	const size = 3
+	interval := time.Millisecond * 50
+	r := NewRollingWindow(size, interval)
+	listBuckets := func() []float64 {
+		var buckets []float64
+		r.Reduce(func(b *Bucket) {
+			buckets = append(buckets, b.Sum)
+		})
+		return buckets
+	}
+	assert.Equal(t, []float64{0, 0, 0}, listBuckets())
+	r.Add(1)
+	assert.Equal(t, []float64{0, 0, 1}, listBuckets())
+	time.Sleep(time.Millisecond * 90)
+	r.Add(2)
+	r.Add(3)
+	assert.Equal(t, []float64{0, 1, 5}, listBuckets())
+	time.Sleep(time.Millisecond * 20)
+	r.Add(4)
+	r.Add(5)
+	r.Add(6)
+	assert.Equal(t, []float64{1, 5, 15}, listBuckets())
+}
+
 func TestRollingWindowReset(t *testing.T) {
 	const size = 3
 	r := NewRollingWindow(size, duration, IgnoreCurrentBucket())