Say why a timestamp was rejected, not just that it was

A value like 2026-08-03T09:15:00 is valid ISO 8601 but rejected here
for lacking an offset; the error now states the actual requirement and
shows accepted examples. Copilot review follow-up.

Author: dkastl

app/controllers/issue_datetimes_controller.rb

@@ -116,7 +116,10 @@ def parse_timestamp(value)
   end
 
   def render_parse_error(param)
-    render json: {errors: ["#{param} must be an ISO 8601 timestamp"]}, status: :unprocessable_entity
+    render json: {
+      errors: ["#{param} must be an ISO 8601 timestamp with a UTC offset, " \
+               'for example 2026-08-03T09:15:00+09:00 or 2026-08-03T00:15:00Z']
+    }, status: :unprocessable_entity
   end
 
   def issue_payload(issue)