add adapter for rss event item

src/redturtle/volto/adapters/configure.zcml

@@ -37,4 +37,11 @@
         provides="plone.restapi.interfaces.IBlockFieldLinkIntegrityRetriever"
         />
   </configure>
+
+  <adapter
+      factory=".rss.EventItem"
+      provides="plone.base.interfaces.syndication.IFeedItem"
+      for="plone.app.contenttypes.interfaces.IEvent
+           plone.base.interfaces.syndication.IFeed"
+      />
 </configure>

src/redturtle/volto/adapters/rss.py

@@ -0,0 +1,19 @@
+from plone.base.interfaces.syndication import IFeed
+from plone.app.contenttypes.interfaces import IEvent
+from Products.CMFPlone.browser.syndication.adapters import DexterityItem
+from zope.component import adapter
+from DateTime import DateTime
+
+
+@adapter(IEvent, IFeed)
+class EventItem(DexterityItem):
+
+    @property
+    def startdate(self):
+        """
+        Same format as other dates in 
+        Products.CMFPlone.browser.syndication.adapters
+        """
+        date = self.context.start.isoformat()
+        if date:
+            return DateTime(date)

src/redturtle/volto/browser/overrides/Products.CMFPlone.browser.syndication.templates.RSS.pt

@@ -56,7 +56,7 @@
     <tal:block tal:repeat="item item/categories">
       <dc:subject tal:content="item">Item</dc:subject>
     </tal:block>
-    <dc:date tal:content="item/published/HTML4|item/modified/HTML4">Published or last modified date if no published date</dc:date>
+    <dc:date tal:content="item/startdate/HTML4|item/published/HTML4|item/modified/HTML4">Published or last modified date if no published date</dc:date>
     <dc:type tal:content="item/context/portal_type">Type</dc:type>
   </item>
 </tal:block>