Quick correction on ical export for one event

app/views/events/show.ics.haml

@@ -1,12 +1,31 @@
 :ruby
   # Create a calendar with an event (standard method)
   cal = Icalendar::Calendar.new
-  tzid = @event.region.tzid
+  tzid = event.region.tzid
+  use_timezone = true
+  if cal.timezones.none? { |t| tzid == t.tzid }
+    # Only add this zone once
+    begin
+      tz = TZInfo::Timezone.get tzid
+      timezone = tz.ical_timezone event.start_time
+      cal.add_timezone timezone
+    rescue TZInfo::InvalidTimezoneIdentifier
+      # No need to add this tz
+      use_timezone = false
+    end
+  end
+
   cal.event do |e|
-    e.dtstamp     = Icalendar::Values::DateTime.new @event.decision_time, tzid: tzid
-    e.uid         = "#{event.id}@#{request.domain}"
-    e.dtstart     = Icalendar::Values::DateTime.new @event.start_time, tzid: tzid
-    e.dtend       = Icalendar::Values::DateTime.new @event.end_time, tzid: tzid
+    e.uid         = "#{@event.id}@#{request.domain}"
+    if use_timezone
+      e.dtstamp   = Icalendar::Values::DateTime.new @event.decision_time, tzid: tzid
+      e.dtstart   = Icalendar::Values::DateTime.new @event.start_time, tzid: tzid
+      e.dtend     = Icalendar::Values::DateTime.new @event.end_time, tzid: tzid
+    else
+      e.dtstamp   = Icalendar::Values::DateTime.new @event.decision_time.localtime
+      e.dtstart   = Icalendar::Values::DateTime.new @event.start_time.localtime
+      e.dtend     = Icalendar::Values::DateTime.new @event.end_time.localtime
+    end
     e.summary     = @event.title
     e.description = to_markdown @event.description.tr '\'', '’'
     e.location    = @event.full_address