Timezones not working ok when booking an appointment after DST change

[Valentina R]

Not all time zones have DST.

A website in the Paris timezone and DST (UTC+2) allows timezone selection. A customer books an appointment 3 weeks ahead (DST change will occur 1 week prior to the appointment) for 13h Reunion time (UTC+4). In the admin dashboard it should show as 10h because now (in standard Paris time UTC+1) there are 3 hours of difference between the two - Reunion stays as UTC+4 all year long.

So, the issues are multiple:

1. The slots available should have shown the 14h slot because the Paris timezone changes, but it shows 13h slot (DST time in Paris - site’s timezone).
2. The slot was booked wrong in WP (Paris) time. It was booked for 11h instead of 10h.
3. The slot was booked wrong in customer emails/my-account: First it said 13h, in the reminder it said 14h.

Reported here.