How to send push notification at exact time (for each each timezone) on exact day, with OneSignal?

Let's say, I want to send at 9am on Friday for each timezone. What is the correct way to set it up with Rest API?

I see that send_after and delivery_time_of_day are separated values. And there are countries in UTC-12 or UTC+12 (and worst case UTC+14). Plus the official website say it's better to set the start time one hour before the first timezone. This makes me feel that if I set it incorrectly, it will send notification to some people one day early or one day late.

How should I do it properly?

For example, New zealand is in UTC+12:00 and goes up to UTC+13:00 in summer.