For a proof-of-concept, I had to install a new Mantis Bug Tracker instance and after finishing the installation, the login screen greeted me with the following warning:
SYSTEM WARNING: date_default_timezone_get(): It is not safe to rely on the system's timezone settings. You are *required* to use the date.timezone setting or the date_default_timezone_set() function. In case you used any of those methods and you are still getting this warning, you most likely misspelled the timezone identifier. We selected 'UTC' for 'UTC/0.0/no DST' instead
The solution is amazingly simple. The only thing that I needed to find out was that this is a PHP error and not a MantisBT error. So after that was settled, I went on and just followed the instructions given by the warning:
You are *required* to use the date.timezone setting
So I edited my php.ini and added the following lines (found in the PHP documentation):
date.timezone = "Europe/Berlin"
date.default_latitude = 52.5194
date.default_longitude = 13.4067
And voilà : Everything works as expected. Note that you might need to refer to the link above in order to get your correct timezone.