diff --git a/mock/mirage_mtime_set.ml b/mock/mirage_mtime_set.ml
index 27f4fbb..6b26bad 100644
--- a/mock/mirage_mtime_set.ml
+++ b/mock/mirage_mtime_set.ml
@@ -1,3 +1,4 @@
 let now = ref 0L
 let tick () = now := Int64.add !now 1L
 let tick_for v = now := Int64.add !now v
+let reset () = now := 0L