These form types should support a way to use the DateTimeImmutable class of PHP 5.5 (only on PHP 5.5+ obviously) for applications using it.

This involves accepting them as input and returning an immutable datetime as model data after the transformation.

The easiest implementation is probably to add a new value for the input option and add a transformer converting the immutable date into a mutable one (so that the norm data stays a DateTime class).

If a new major version of Symfony bumps the requirement to PHP 5.5+, the normalized data could then be switched to the DateTimeImmutable class to simplify the logic (which is currently cloning the DateTime to avoid modifying the original one.