Hi Allison -
I don't believe that Cvent currently has a field that reflects all the different time zones so more than likely, you would need to add the question and include some pre-determined selections.
I guess it also begs the question of why are you asking? Is it so that calendar invitations, agenda, and registration information can reflect accordingly? If so, you can enable the option to allow the attendees to adjust the time zones to reflect their device's local time zone. Here's an article to walk through how to set that up:
https://support.cvent.com/s/communityarticle/How-can-my-invitees-view-my-event-in-their-devices-time-zone
If you are just trying to collect that information to better set up your sessions, we've asked it in a couple of different ways. We've added sessions (by regions) and asked the registrants to select their session based on their preferred/attending from region or we've also asked it as a question (also based on their region) - What region will you be participating from? Responses include: Americas (Eastern/Central/Mountain/Pacific), etc.
I'd be curious to know why you might need each attendee's specific time zone.
Hope this helps!
------------------------------
Rebecca Yousif
Senior Administrator - Event Technology
------------------------------