To me it happened from one moment to the other. My Mac has forgotten all timezones i.e. "sudo systemsetup listtimezones" returns an empty list, the default timezone is GMT and my mac thinks I am in Cupertino,CA. Timezone in Preferences shows as "Unknown". I wouldn't care but it's messing up all my Outlook appointments as well as single-sign-ons (e.g. can't login to web ms teams anymore).

I had the same problem and managed to fix it - over at apple.stackexchange is a great thread and a description how I fixed it https://apple.stackexchange.com/a/468997/102293