Set the viewer timezone properly on Calendar event RecurrenceSet objects

Summary: Ref T11801. In some cases, this could lead to us failing to generate the first recurrence in a series.

Test Plan: Imported `weekly.ics` (from D16974) and saw an event correctly occur on Aug 18, with my local timezone set to "America/Los_Angeles".

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T11801

Differential Revision: https://secure.phabricator.com/D16975

src/applications/calendar/storage/PhabricatorCalendarEvent.php

@@ -1024,6 +1024,10 @@ final class PhabricatorCalendarEvent extends PhabricatorCalendarDAO
 
     $set = new PhutilCalendarRecurrenceSet();
 
+    if ($this->viewerTimezone) {
+      $set->setViewerTimezone($this->viewerTimezone);
+    }
+
     $rrule = $this->newRecurrenceRule();
     if (!$rrule) {
       return null;