Fix explore prompt appearing because of posts being received out of order (#27211)

app/javascript/mastodon/features/home_timeline/index.jsx

@@ -39,8 +39,17 @@ const getHomeFeedSpeed = createSelector([
 ], (statusIds, pendingStatusIds, statusMap) => {
   const recentStatusIds = pendingStatusIds.size > 0 ? pendingStatusIds : statusIds;
   const statuses = recentStatusIds.filter(id => id !== null).map(id => statusMap.get(id)).filter(status => status?.get('account') !== me).take(20);
-  const oldest = new Date(statuses.getIn([statuses.size - 1, 'created_at'], 0));
-  const newest = new Date(statuses.getIn([0, 'created_at'], 0));
+
+  if (statuses.isEmpty()) {
+    return {
+      gap: 0,
+      newest: new Date(0),
+    };
+  }
+
+  const datetimes = statuses.map(status => status.get('created_at', 0));
+  const oldest = new Date(datetimes.min());
+  const newest = new Date(datetimes.max());
   const averageGap = (newest - oldest) / (1000 * (statuses.size + 1)); // Average gap between posts on first page in seconds
 
   return {