need some suggestion about UTC Timezone

hi

I need a field named "Timezone" in the Join form. I want users in different timezone can login online at the same  time.

I found "Site Timezone" in the "Basic Settings" .

But I can not find Timezone in the "Predefined Values".

I am wandering if I must create a predefined value named Timezone manually.

thanks 

Events module uses UTC time by default for event date start/end.

It maybe you can use Events module for you purposes.

Thanks a lot.