diff --git a/storage/local/series.go b/storage/local/series.go
index f17a92b3e..75545830d 100644
--- a/storage/local/series.go
+++ b/storage/local/series.go
@@ -288,7 +288,7 @@ func (s *memorySeries) maybeCloseHeadChunk() bool {
 func (s *memorySeries) evictChunkDescs(iOldestNotEvicted int) {
 	lenToKeep := chunkDescEvictionFactor * (len(s.chunkDescs) - iOldestNotEvicted)
 	if lenToKeep < len(s.chunkDescs) {
-		s.savedFirstTime = s.FirstTime()
+		s.savedFirstTime = s.firstTime()
 		lenEvicted := len(s.chunkDescs) - lenToKeep
 		s.chunkDescsOffset += lenEvicted
 		s.persistWatermark -= lenEvicted
@@ -514,7 +514,7 @@ func (s *memorySeries) head() *chunk.Desc {
 // firstTime returns the timestamp of the first sample in the series.
 //
 // The caller must have locked the fingerprint of the memorySeries.
-func (s *memorySeries) FirstTime() model.Time {
+func (s *memorySeries) firstTime() model.Time {
 	if s.chunkDescsOffset == 0 && len(s.chunkDescs) > 0 {
 		return s.chunkDescs[0].FirstTime()
 	}
diff --git a/storage/local/storage.go b/storage/local/storage.go
index 72332bad1..10b825adb 100644
--- a/storage/local/storage.go
+++ b/storage/local/storage.go
@@ -657,7 +657,7 @@ func (s *MemorySeriesStorage) metricForRange(
 ) (model.Metric, *memorySeries, bool) {
 	series, ok := s.fpToSeries.get(fp)
 	if ok {
-		if series.lastTime.Before(from) || series.FirstTime().After(through) {
+		if series.lastTime.Before(from) || series.firstTime().After(through) {
 			return nil, nil, false
 		}
 		return series.metric, series, true
@@ -1264,7 +1264,7 @@ func (s *MemorySeriesStorage) maintainMemorySeries(
 	if iOldestNotEvicted == -1 && model.Now().Sub(series.lastTime) > headChunkTimeout {
 		s.fpToSeries.del(fp)
 		s.numSeries.Dec()
-		s.persistence.archiveMetric(fp, series.metric, series.FirstTime(), series.lastTime)
+		s.persistence.archiveMetric(fp, series.metric, series.firstTime(), series.lastTime)
 		s.seriesOps.WithLabelValues(archive).Inc()
 		oldWatermark := atomic.LoadInt64((*int64)(&s.archiveHighWatermark))
 		if oldWatermark < int64(series.lastTime) {
@@ -1325,7 +1325,7 @@ func (s *MemorySeriesStorage) writeMemorySeries(
 		chunks[i] = cd.C
 	}
 
-	if !series.FirstTime().Before(beforeTime) {
+	if !series.firstTime().Before(beforeTime) {
 		// Oldest sample not old enough, just append chunks, if any.
 		if len(cds) == 0 {
 			return false
diff --git a/storage/local/storage_test.go b/storage/local/storage_test.go
index 73804fedc..9507da342 100644
--- a/storage/local/storage_test.go
+++ b/storage/local/storage_test.go
@@ -69,7 +69,7 @@ func TestMatches(t *testing.T) {
 			t.Fatal("could not retrieve series for fp", fp)
 		}
 		storage.fpLocker.Lock(fp)
-		storage.persistence.archiveMetric(fp, s.metric, s.FirstTime(), s.lastTime)
+		storage.persistence.archiveMetric(fp, s.metric, s.firstTime(), s.lastTime)
 		storage.fpLocker.Unlock(fp)
 	}
 
@@ -1276,7 +1276,7 @@ func testEvictAndPurgeSeries(t *testing.T, encoding chunk.Encoding) {
 	if err != nil {
 		t.Fatal(err)
 	}
-	s.persistence.archiveMetric(fp, series.metric, series.FirstTime(), lastTime)
+	s.persistence.archiveMetric(fp, series.metric, series.firstTime(), lastTime)
 	archived, _, _ := s.persistence.hasArchivedMetric(fp)
 	if !archived {
 		t.Fatal("not archived")
@@ -1317,7 +1317,7 @@ func testEvictAndPurgeSeries(t *testing.T, encoding chunk.Encoding) {
 	if err != nil {
 		t.Fatal(err)
 	}
-	s.persistence.archiveMetric(fp, series.metric, series.FirstTime(), lastTime)
+	s.persistence.archiveMetric(fp, series.metric, series.firstTime(), lastTime)
 	archived, _, _ = s.persistence.hasArchivedMetric(fp)
 	if !archived {
 		t.Fatal("not archived")