Except if you do it by GMT without accounting for daylight savings time, then some of us have to change it twice a year. Adding a tickbox for daylight savings time would help, but are there different dates to start/end it in different countries?

And some people don't know GMT offsets in their head at all - in fact, I'd say the majority of people I know here in the US don't. If they don't deal in computers or technical stuff, and don't know anyone in that time zone, the odds are they don't know the GMT offset.