Short answer
HubSpot's KB: schedule a marketing email at a specific time in each recipient's time zone; it sends whenever that time next occurs there. You can't use it with A/B tested emails, and send-to-more isn't available, so clone the email for extra recipients.
1. In HubSpot
The relevant screen in my demo portal.
2. Worth knowing
If the time has already passed in a zone on the scheduled day, that zone receives it the next time the time comes around.