rrule (TZInfo::TimezoneTransitionInfo)

    # File lib/icalendar/tzinfo.rb, line 67
            def rrule
                start = local_start.to_datetime
                # this is somewhat of a hack, but seems to work ok
                [sprintf(
                        "FREQ=YEARLY;BYMONTH=%d;BYDAY=%d%s",
                        start.month,
                        ((start.day - 1)/ 7).to_i + 1,
                        start.strftime("%a").upcase[0,2]
                        )]
            end