Kestrel TimeZone implementation does not have any histric TimeZone changes. It only applies rule of year 2000. There are time zone like Australia/Sydney -- has exception on DST on year 2000 only --, or like Iran -- that changes DST rule every year. Without holding histrical data, like UNIX tz do, it's not possible to deal with these time zones.
We have to always update timezone data every year, if the one we have now is correct. This is not good implementation. Also, all the timezone data is hardcoded in TimeZone data. This is lack of maintainancability. TimeZone data will change a lot year by year. Since it's just each country local data.