The HTML5 datepicker component now works in most browsers and the minimum and maximum control attributes are fully implemented. The correct format is yyyy-mm-dd, as described in the mozilla documentation.
A simple example adapted from w3school:
Show a Date Control
Birthday:
Note: type="date" is not supported in Safari or Internet Explorer 11 (or earlier).
As supplementary information, from the HTML5 specification documentation.
The min attribute, if specified, must have a value that is a valid
date string. The max attribute, if specified, must have a value that
is a valid date string.
...is intended to be computer-readable and consistent irrespective of
the user's locale. Dates, for instance, are always written in the
format "YYYY-MM-DD", as in "2003-02-01".