HTML input fields have native placeholders. They are often used to show the correct format for the field, like in the example above.
I see the placeholder, kinda get it, then start typing and boom, the placeholder is gone, and I don't remember what it was. Was it year-dash-month…? Or slash? Or period? I have to delete my input to see the placeholder again.
We don't remember something explicitly until we know we must. And I only know I must remember when I start typing and see the placeholder gone.
A nicer way to handle this would be showing the relevant portion of the placeholder even as I type.
Or just accept ALL sensible formats and don't make the user think.