diff --git a/src/components/TimelineActionCard/TimelineActionCard.js b/src/components/TimelineActionCard/TimelineActionCard.js
index 1137b19..078e24c 100644
--- a/src/components/TimelineActionCard/TimelineActionCard.js
+++ b/src/components/TimelineActionCard/TimelineActionCard.js
@@ -52,11 +52,12 @@ export default function TimelineActionCard({ type, datetime, by, content }){
 }
 
 TimelineActionCard.propTypes = {
+    /** The type of card to render. */
     "type": PropTypes.oneOf([
         "edit",
         "reply_to_user"
     ]),
-    /** ISO 8601 formatted time string. */
+    /** Any string, number or Date object understood by React Relative Time. See https://github.com/aharshac/react-relative-time#readme. */
     "datetime": PropTypes.string.isRequired,
     /** The name of the person who added the action. */
     "by": PropTypes.string.isRequired,