diff --git a/neuralnetworks/1.3/types.t b/neuralnetworks/1.3/types.t
index a973923d8d..d4351ec8d7 100644
--- a/neuralnetworks/1.3/types.t
+++ b/neuralnetworks/1.3/types.t
@@ -552,6 +552,19 @@ safe_union OptionalTimePoint {
     uint64_t nanoseconds;
 };
 
+/**
+ * Optional timeout duration measured in nanoseconds.
+ */
+safe_union OptionalTimeoutDuration {
+    /** No time point provided. */
+    Monostate none;
+
+    /**
+     * Timeout duration measured in nanoseconds.
+     */
+    uint64_t nanoseconds;
+};
+
 /**
  * Return status of a function.
  */