Why is Wordpress saving the wrong date?

Question

WordPress

When saving the date in any part of the admin panel, it is saved a day earlier (let's say you chose 05/07/2021, but 05/06/2021 was saved).
The site is French, in the wp setting is their time zone (+2).
If you put UTC0, then saving is normal. But then the time on the site will be wrong.
Why is that?

Answer 1

What will showdate_default_timezone_get();