the DateTimeInterface itself is immutable

Do you mean this interface is immutable by design or it's immutable because by coincidence all the methods it has do not care whether a concrete implementation is mutable or immutable?

im not sure it's a job for Symfony

I know the conversation what's framework's job is hard. However it always comes down to the specific problem, in this case DateTime vs DateTimeImmutable. Mutable DateTime is flawed and it should have never been introduced (wrong decision back in PHP 5.5 where the PHP core did not have the courage to revert a bad decision introduced in 5.2). In my opinion the lead tools and their maintainers should have the courage to make hard and what's more the right decisions because who else will be? Some random John Doe from China? If Symfony switched to the DateTimeImmutable it would be a huge signal for the PHP world, hopefully leading to the the removal of mutable DateTime from the core.

Symfony always instantiate DateTimeImmutable now (except for cases where it is explicitly requested to provide a DateTime object).

But this does not mean that Symfony cannot be more flexible in the kind of input it accepts.

@stof this is what I want to point out - I know frameworks should be flexible but they should also show a good practices, be a role model. Right now when we have DateTimeInterface TH we show that we are fine with DateTime. I think Symfony could go one step further and show DateTime is wrong by not accepting it as a method argument.