diff --git a/docs/ApiValidationRules.md b/docs/ApiValidationRules.md
index 9cf2073..701a681 100644
--- a/docs/ApiValidationRules.md
+++ b/docs/ApiValidationRules.md
@@ -68,6 +68,18 @@ If a validation rule fails the call should return 400 (Bad request) including an
 }
 ```
 
+### Time values
+
+Date and date+time values are specified as text formatted in [RFC 3339](https://www.rfc-editor.org/rfc/rfc3339#section-5.6), e.g.
+
+```json
+{
+    "created" : "2024-01-27T18:00:21Z"
+}
+```
+
+Usually the "Z" is missing at the end indicating local time. 
+
 ## /shipcall POST
 
 1. The call may only be performed by a user belonging to participant group type BSMD.