Easy (for the new product or .Threads anyway), define:
lol... It's what I use on all my sites to define the timezone; works fine with DST and offsets; best of all it uses standard PHP
So you could have all of the users list their timezone as above (or just have a dropdown with the value with the item above) then it'd load their timezone by what they choose so long as the page has the putenv before the time... I am however unsure how reliabele the date system is for adjusting the timezone during DST lol
