Courtesy of the RTM Admin Team, I've found a handy tool and instantly thought of the most multi-national team I ever gamed with >_>.
You enter the date and time zone/location for an event into the form and push calculate fixed time. It then gives a page that lists the equalivulent time everywhere else, and the address bar link is copy/paste-able. So if you're planning something at 8pm your time, you enter that, and the link provides conversions into everyone else's local times; all calculated based on UTC/GMT.
When you remove the arithmetic from figuring out the UTC/GMT offsets, it don't get any simpler ;). Of course as always one still has to find a location nearest you on the map \o/.