What is IST? Internation Standard Time? To me that would be GMT but GMT is oblivious to daylight savings and as governments can change when and where DST applies there is no international standard.
My bet is you'll have to do a web scrape of some country's ministry site to get what it is now. I see British Columbia, Canada is about to dispose of DST since there is no to little savings by using such a system.
-> Are you aware how Microsoft deals with DST?