Getting 'Unknown or bad timezone ' when trying to login into admin

Just tried to login into the admin interface of grav and got the following error message after login-in the admin:

DateInvalidTimeZoneException
DateTimeZone::__construct(): Unknown or bad timezone (Asia/Choibalsan)

More info:

It never happened before, and I haven't made any changes since last time I logged in. I have no idea how to fix this.

Sounds like you had PHP config changes. And this exception is only available since PHP 8.0, on which Grav 1.7 doesn't work and 1.8 is still in beta

Ok thanks. I how should I fix this ?

Find out if/who changed your PHP config and revert. Talk to your hosting provider maybe