HTML5 pattern for formatting input box to take date mm/dd/yyyy?

Easiest way is use read only attribute to prevent direct user input:

 <input class="datepicker" type="text" name="date" value="" readonly />

Or you could use HTML5 validation based on pattern attribute.
Date input pattern (dd/mm/yyyy or mm/dd/yyyy):

<input type="text" pattern="\d{1,2}/\d{1,2}/\d{4}" class="datepicker" name="date" value="" />