I see what you've done - clever - if you're making it so that it checks on the index.php file, it might be a good idea to add the hi_timezoneoffset template into the templateused cache in index.php OR instead of using templates, just add the contents of hi_timezoneoffset in the <head> section of forumhome. Otherwise, it's an extra unnecessary query in forumhome.
Also, you can achieve the same thing by using templates - instead of editing global.php, add the same code to phpinclude (including the contents of the PHP file - this whole hack could be template based).

Great idea. I'm at work, but when I get home I'll play around with it.