Remembering which country is where is standard in school, roughly estimating timezones from that is easy (US varies from -8 to -4, most of europe is 0 or +1 or +2, as is most of Africa, Russia is +3, middle east is around +3 and +4, china is +8, just like singapore and japan; australia, new zealand are between +10 and +12; I have no idea what the actual data is, just saying what I remember based on knowing the world map)
As everyone knows GMT, and everyone knows the world map, the issue is solved.
Google calendar is even smart enough to "translate" an event to your timezone. If I get an email with an event in a different timezone and put it on your Google Calendar it actually goes on your calendar in your local time zone. This screwed me up once when I moved to the other side of the world and forgot to update my timezone in Google.