diff --git a/lib/storage/storage.go b/lib/storage/storage.go
index ee8e53ca7b..a4106901c4 100644
--- a/lib/storage/storage.go
+++ b/lib/storage/storage.go
@@ -50,9 +50,9 @@ type Storage struct {
 	slowPerDayIndexInserts uint64
 	slowMetricNameLoads    uint64
 
-	path            string
-	cachePath       string
-	retentionMonths int
+	path           string
+	cachePath      string
+	retentionMsecs int64
 
 	// lock file for exclusive access to the storage on the given path.
 	flockF *os.File
@@ -118,11 +118,10 @@ func OpenStorage(path string, retentionMsecs int64) (*Storage, error) {
 	if retentionMsecs <= 0 {
 		retentionMsecs = maxRetentionMsecs
 	}
-	retentionMonths := (retentionMsecs + (msecsPerMonth - 1)) / msecsPerMonth
 	s := &Storage{
-		path:            path,
-		cachePath:       path + "/cache",
-		retentionMonths: int(retentionMonths),
+		path:           path,
+		cachePath:      path + "/cache",
+		retentionMsecs: retentionMsecs,
 
 		stop: make(chan struct{}),
 	}
@@ -473,8 +472,9 @@ func (s *Storage) startRetentionWatcher() {
 }
 
 func (s *Storage) retentionWatcher() {
+	retentionMonths := int((s.retentionMsecs + (msecsPerMonth - 1)) / msecsPerMonth)
 	for {
-		d := nextRetentionDuration(s.retentionMonths)
+		d := nextRetentionDuration(retentionMonths)
 		select {
 		case <-s.stop:
 			return