diff --git a/specification/types.md b/specification/types.md
index bc9eef6b..18c49c5d 100644
--- a/specification/types.md
+++ b/specification/types.md
@@ -28,7 +28,7 @@ Structured data, presented however is idiomatic in the implementation language,
 
 ### Datetime
 
-A language primitive for representing a date and time, including timezone information.
+A language primitive for representing a date and time, optionally including timezone information. If no timezone is specified, the date and time will be treated as UTC.
 
 ### Evaluation Details