events: Add DTSTAMP to ICS feed entries

mpicms/events/models.py

@@ -4,6 +4,7 @@
 
 from django.db import models
 from django.core.exceptions import ValidationError
+from django.utils import timezone
 from django.utils.translation import gettext_lazy as _
 
 from wagtail.admin.panels import FieldPanel, MultiFieldPanel
@@ -134,7 +135,9 @@ def get_ics(self, external=False):
                 end=event.end,
                 description=event.search_description,
                 url=event.full_url,
-                location=event.room
+                location=event.room,
+                # ics 0.7 serializes "created" as DTSTAMP, which RFC 5545 requires.
+                created=event.last_published_at or event.latest_revision_created_at or timezone.now()
             )
             if type(event.start) is date:
                 e.make_all_day()