@Jon1, @mansuetus I think I know what caused this:
I just reset my phone to start again (even google-free-er than before; because somehow some google (anti)services crept in…) and didn’t enter my Wi-Fi password when setting up. I then got to a screen I didn’t see the last time I set up asking me to change date and time “if necessary”.
The date and time was set to 1.1.1970 11:11.
I guess maybe you didn’t have internet connection when you set up your phone so since the phone believed then that it was 1970 it now believes you had your phone since then.