The previous implementation had the following bugs:
- DST was truncated to end of October
- DST start was off by one week in years where March 1st is a Monday
- DST start was off by one second (1:59:59 -> 2:00:00 -> 3:00:01 instead of the correct 1:59:59 -> 3:00:00 -> 3:00:01)
Tested against tzdata 2009g.