date/timezone issue

[speedy33417]

I'm working on a function that processes $time_gmt and $timezone and returns $time.
$time_gmt is true GMT time and the value of $timezone can be "GMT", "EST", "CET", etc. and is user selected upon registration and stored in a db.
$time is $time_gmt in the user's selected time zone.

My question is how do I generate a true GMT time?

[code]echo gmdate('l, Y F j, H:i:s \G\M\T', time());[/code]

It gives me 16:28, but it's 17:28 in London now...