When you're setting your campaign's end date and time, you're also prompted to select a timezone. Any timezone will work – we've helped organizers run campaigns in all of them!
We suggest choosing the timezone where you're based, or where most of your audience is based. (Typically, that's the same timezone!)
If you'd like to have your message blast out at different times across timezones, you'll need to set up a new campaign for each one. Since there's one message blast per campaign, be sure to set up multiple campaigns to stagger the end times of each message blast.