MyZones helps you configure the timezones you are most interested in and then run it. You can go forward or backwards in time if you are trying to schedule a date in the future, say.
You can configure to show the timezones you are interested in. As long as the timezone on your machine is correct and your computer's clock is on time, then it should show the right time for that timezone. The pytz (apt-get install python-tz for debian users) module is very complete (and correct) even has historical timezones that don't exist anymore.
If you live in the southern hemisphere like I do, you probably constantly struggle to figure out what time it is in the northern hemisphere. This is because, when daylight savings time comes around the north jumps forward and we jump back (or vice-versa). To make things even more complicated, the date and hour that the time changes is usually different. There will be one or more weeks where time difference is X hours. Add the classic "off by one" error or a little dyslexia and your bound to get the time wrong and wake somebody up at 8:00 in the morning.
Under Linux create a file called ~/.myzones.ini or ~/myzones.ini and it'll use that file for configuration. Under Windows you would put the file it under something like 'c:\Documents and Settings\scott\My Documents'.
The configuration file looks something like this
In the config section, you can put in some configuration information, of which there is currently only one thing to configure.
For the timezones section you can give any names on the left hand side and the time zone (one of the Common Zone Names ) If you want the timezones to appear in a certain order you need to prepend the name with 1|, 2|, etc., and is thus recommended.