diff --git a/specification/trace/api.md b/specification/trace/api.md
index 7736aa5f8b5..de0100a0d4d 100644
--- a/specification/trace/api.md
+++ b/specification/trace/api.md
@@ -428,7 +428,7 @@ An `Event` is defined by the following properties:
 
 - (Required) Name of the event.
 - (Optional) [`Attributes`](../common/common.md#attributes).
-- (Optional) Timestamp for the event.
+- (Optional) Timestamp for the event. If not provided, the current time when the event is added MUST be used.
 
 The `Event` SHOULD be an immutable type.