`Safe\DateTimeImmutable` is not compatible with `\DatePeriod`

[Kharhamel]

As seen in #227, DatePeriod can take Safe\DateTimeImmutable as parameter but will mess its internal state and overall won't work correctly. The only workaround is to switch batch to \DateTimeImmutable with ->getInnerDateTime().

It would be nice to have a way to enforce this workaround, with a phpstan rule for example.