I'm surprised no browser has a HTML based implementation which has been copied/extended in styling.

Just let me type the date in the user's local format and put in the dashes/slashes with a regex (and remove them again before applying insert/Backspace/Delete). Almost always this is so messed up and broken. And slow.

Sigh. You anonymous code authors are always working to bring the user down, even when there is a 1000000:1 ratio between them and you.