Without additional configuration, the time shown in signatures, etc. will be in UTC as this is saved into the page content and isn't processed per-user.
That's odd then - UTC is 2007-01-28 02:30, my local time is GMT+10 (2007-01-28 12:30) and the time appearing in my signatures is 2007-01-28 13:30.
I say "without additional configuration" because there are some configuration variables which deal with timezone offsets on the server side, and these will alter various aspects of date/time handling.
Ah yes, I had a look through DefaultSettings.php and I see there's a variable for signature times. This was set to null (meaning the OS default) however the OS was configured for GMT+10 and the signatures were appearing as GMT+11 ("EST"). I changed this to "GMT" in LocalSettings.php and all the signatures are now correctly displaying GMT times.
Thanks for your help!
Cheers, Adam.