I think they're actually quite sophisticated about it - they look at the time zone according to the shopper's browser. So they're looking at local-to-the-shopper.
Yep! We try to localize the time.
So the fraudsters in Vietnam are trying to time it for 2am to 4am in the local market of the site they're defrauding?
Not quite...an order is more likely fraudulent when it was placed at 2-4am local time. Local time for the fraudster (in Vietnam or wherever else), not local time for the site they're defrauding.