I haven’t used Mambo, but I know Joomla split off of it, so they must be very similar. There are two seperate offset values in the Joomla locale settings. One is for the users timezone in the form of offset from GMT. The other says “server offset”.
These are stored in the configuration.php file like this:
$mosConfig_offset = '3';
$mosConfig_offset_user = '-4';
They’re not quite right in my case. I think they need to be 2 and -5, but you can’t seem to change serveroffset in the admin. I just messed with it until the popup info window displayed the correct time here: http://benconley.net/images/articleimg/joomlaConfig.gif
While there is no modifier greater than 12, you will be able to get to the right number using the correct combination.