If you know what timezone they're in, try to work out what time in your own timezone your free, then tell the other person to do the same. Tell them what times in your timezone and the other can search up what the time is in their own timezone. If there is a specific time when you are both free, use that.
Ex:
You live in the U.S. but you have a meeting with someone in the U.K. You're only free at 4:00pm-6:00pm and they're only free from 10:00am-11:00am. 11:00am in England is 4:00pm in the U.S. so do at that time!
Hope this helps!