Steps

1. Set in php.ini date.timezone to e.g.: date.timezone = 'Europe/Warsaw'
2. Run phpunit
3. See failures in expected vs. actual time strings in about 12 tests

         -'2020-12-11T08:26:43+00:00'
         +'2020-12-11T09:26:43+01:00' 

Expected result

Tests pass regardless of local timezone setting.

Note

By default the timezone is set to Europe/Berlin and then test don't fail, which is peculiar, since technically this is also CET timezone.