Below pattern perfectly works in case of leap year and as well as with normal dates. The date format is : YYYY-MM-DD
<input type="text" placeholder="YYYY-MM-DD" pattern="(?:19|20)(?:(?:[13579][26]|[02468][048])-(?:(?: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))|(?:[0-9]{2}-(?:0[1-9]|1[0-2])-(?:0[1-9]|1[0-9]|2[0-8])|(?:(?!02)(?:0[1-9]|1[0-2])-(?:29|30))|(?:(?:0[13578]|1[02])-31)))" class="form-control " name="eventDate" id="" required autofocus autocomplete="nope">
I got this solution from http://html5pattern.com/Dates. Hope it may help someone.