Since users are all-over-the-world, GMT would be the best solution.
http://time.greenwich2000.com/

World Time is set Greenwich Mean Time - GMT.

How GMT is being used today...
Your email box is full of GMT references that you probably didn't know about!
Do your emails display something like:

"Tue, 29 Oct 1996 16:41:46 -0500 (EST)" and ever wondered what the -0500 (or equivalent number) means?

It is the time offset from GMT

I.E. In this example EST (Eastern Standard Time is 5 hours behind GMT)

Wonder what those cookies are?
They work on GMT and only GMT (Netscape description)

Preliminary Specification of HTTP Cookies

Today the US Space Shuttle uses Greenwich Mean Time.