diff --git a/cmd/aggregator/backfill.go b/cmd/aggregator/backfill.go
index c27f1db..e3c2fe0 100644
--- a/cmd/aggregator/backfill.go
+++ b/cmd/aggregator/backfill.go
@@ -58,8 +58,8 @@ func Backfill(ctx context.Context, collectorAddrs []string, cache *Cache) {
 
 	for range collectorAddrs {
 		r := <-ch
-		mergeDump(r.fine, fineByTS)
-		mergeDump(r.coarse, coarseByTS)
+		mergeDump(r.fine, fineByTS, time.Minute)
+		mergeDump(r.coarse, coarseByTS, 5*time.Minute)
 	}
 
 	mergeStart := time.Now()
@@ -114,9 +114,10 @@ func dumpCollector(ctx context.Context, addr string) (fine, coarse []st.Snapshot
 
 // mergeDump adds all snapshots from one collector's dump into the per-timestamp
 // accumulator map. Multiple collectors' entries for the same timestamp are summed.
-func mergeDump(snaps []st.Snapshot, byTS map[int64]map[string]int64) {
+// granularity should match the ring bucket size (time.Minute for fine, 5*time.Minute for coarse).
+func mergeDump(snaps []st.Snapshot, byTS map[int64]map[string]int64, granularity time.Duration) {
 	for _, snap := range snaps {
-		ts := snap.Timestamp.Unix()
+		ts := snap.Timestamp.Truncate(granularity).Unix()
 		m := byTS[ts]
 		if m == nil {
 			m = make(map[string]int64, len(snap.Entries))