Before PHP 8.0.10, the time is lost when datetime->modify() is used
with weekdays. However, with PHP 8.0.10, the time is kept.
In consequence, the test_yearly_every_20th_monday_forever method
started failing.
For fixing it, the offset has been removed and the time has been set
to 00:00, so the behaviour will be the same regardless the PHP
version.