diff --git a/db.go b/db.go
index 40025bdbc..bbd46c301 100644
--- a/db.go
+++ b/db.go
@@ -184,7 +184,7 @@ func newDBMetrics(db *DB, r prometheus.Registerer) *dbMetrics {
 	})
 	m.startTime = prometheus.NewGaugeFunc(prometheus.GaugeOpts{
 		Name: "prometheus_tsdb_lowest_timestamp",
-		Help: "Lowest timestamp value stored in the database.",
+		Help: "Lowest timestamp value stored in the database. The unit is decided by the library consumer.",
 	}, func() float64 {
 		db.mtx.RLock()
 		defer db.mtx.RUnlock()
diff --git a/head.go b/head.go
index 1988ac372..90aba0b94 100644
--- a/head.go
+++ b/head.go
@@ -137,13 +137,13 @@ func newHeadMetrics(h *Head, r prometheus.Registerer) *headMetrics {
 	})
 	m.maxTime = prometheus.NewGaugeFunc(prometheus.GaugeOpts{
 		Name: "prometheus_tsdb_head_max_time",
-		Help: "Maximum timestamp of the head block.",
+		Help: "Maximum timestamp of the head block. The unit is decided by the library consumer.",
 	}, func() float64 {
 		return float64(h.MaxTime())
 	})
 	m.minTime = prometheus.NewGaugeFunc(prometheus.GaugeOpts{
 		Name: "prometheus_tsdb_head_min_time",
-		Help: "Minimum time bound of the head block.",
+		Help: "Minimum time bound of the head block. The unit is decided by the library consumer.",
 	}, func() float64 {
 		return float64(h.MinTime())
 	})