[kevincox]

The problem isn't that GMT changes, it is that the local time changes.

If I schedule an event in 1 year at 08:00 in MyCityTimezone and you store that the event occurs at 13:00 GMT then 3 months later MyCity decides that they aren't doing Daylight Savings Anymore your GMT time is now wrong. My event must still occur at 08:00 which is now 14:00 (or whatever) GMT.

"Just" storing it in GMT isn't enough. You probably do want to do that but you need to store the local time as well and update GMT to match if timezones change.

This is very frequently done wrong because timezones don't change often so the "Just store GMT" mostly works. But if you are writing a high quality system you need to deal with these cases.