I think the previous formats were meant to be human-readable and not something that you could pass todate() orDateTimeInterface::format(). And I think not having the reader to know all the format options is a good thing. But can we somehow end up with something more clear than we have right now to avoid confusion?

The Date constraint accepts only one format as mentioned in the example, $birthday it should be a string "Y-m-d" formatted value.
There is no other option to use it other than H:i:s for Time() and Y-m-d for Date().

In the code examples we mentioned that those were the only valid format ... but I agree that we could be explicit about this in the explanation too. I added some real example too in case any reader needs help with the PHP date formatting letters.

@adnen-chouibi thanks and congrats on your first Symfony Docs contribution 🎉