diff --git a/src/ExperimentalInternetComputer.mo b/src/ExperimentalInternetComputer.mo
index d7e158be..3a3d4c2d 100644
--- a/src/ExperimentalInternetComputer.mo
+++ b/src/ExperimentalInternetComputer.mo
@@ -82,4 +82,10 @@ module {
   /// ```
   public let performanceCounter : (counter : Nat32) -> (value: Nat64) = Prim.performanceCounter;
 
+  /// Returns the time (in nanoseconds from the epoch start) by when the update message should
+  /// reply to the best effort message so that it can be received by the requesting canister.
+  /// Queries and non-best-effort update messages return zero.
+  ///
+  public func replyDeadline() : Nat = Prim.nat64ToNat(Prim.replyDeadline());
+
 }