Good point.
How to Customize HTML5 Date Field Date Format | HTML Form Guide suggests:
<input type="text" name="input" placeholder="YYYY-MM-DD" required
pattern="(?:19|20)\[0-9\]{2}-(?:(?:0\[1-9\]|1\[0-2\])-(?:0\[1-9\]|1\[0-9\]|2\[0-9\])|(?:(?!02)(?:0\[1-9\]|1\[0-2\])-(?:30))|(?:(?:0\[13578\]|1\[02\])-31))"
title="Enter a date in this format YYYY-MM-DD"/>
More on patterns is here: HTML input pattern Attribute
I’m sure I found a date pattern that was simpler, but google is not being friendly tonight.
Also, I think for a pattern to work, you have to have the form tag.