Whenever I have serious communication with someone from a different time zone, I always quote my GMT (if needed), so if they're smart enough, they know what time I'm at. This is especially good for international phone calls, which you don't want to get at 3am.
The theory of time zones, and how to see what time zone anyone is in, at any given time of day, is a fairly simple idea, and it amazes me that so many people have so much trouble with it.
Anyone with a globe can figure it out, and you don't need a calculator.
With that said, the biggest downfall is that some places use daylight saving time, others do not, but even then, you're only going to be out by one hour either way.
BTW, I'm at GMT-8
