diff --git a/core/src/time_ns.ml b/core/src/time_ns.ml
index 003e9b4..e1f039e 100644
--- a/core/src/time_ns.ml
+++ b/core/src/time_ns.ml
@@ -22,6 +22,7 @@ let epoch = Span.zero
 let add = Span.( + )
 let sub = Span.( - )
 let diff = Span.( - )
+let since = diff @@ now ()
 let abs_diff t u = Span.abs (diff t u)
 let max = Span.max
 let min = Span.min
diff --git a/core/src/time_ns_intf.ml b/core/src/time_ns_intf.ml
index 89b937b..6568179 100644
--- a/core/src/time_ns_intf.ml
+++ b/core/src/time_ns_intf.ml
@@ -396,6 +396,9 @@ module type Time_ns = sig
   (** overflows silently *)
   val diff : t -> t -> Span.t
 
+  (** overflows silently *)
+  val since : t -> Span.t
+
   (** overflows silently *)
   val abs_diff : t -> t -> Span.t