Typehint to DateTimeImmutable instead of DateTimeInterface

[Roshyo]

For some (obscure) reasons, the DateTimeInterface is missing some methods, such as add() modify(). And since the current implementation is only returning DateTimeImmutable, I think we should typehint it to that, so StaticAnalysis tools will stop complaining.

[stof]

the reason is not obscure actually: DateTime::add and DateTimeImmutable::add cannot be used in the same way transparently (with DateTime::add, the current object is mutated, so your code might be ignoring the return value)

[Roshyo]

Ha okay, I understand why now. Thanks for the clarification ❤️