I usually mention my time zone when I say the time. Sometimes I'll convert it into the other three United States time-zones and say all four times. If you don't live in the United States, I can't really help you then; mainly because I don't really know the time-zones beyond it.
So I'll usually say something like: I got to go at 6:00 Central time (4:00 Pacific, 5:00 Mountain, 7:00 Eastern)
Sometimes its fun to try and guess where people live by their time-zones. In game I've met people from all across North America, and even Australia and Britain, it's pretty cool.