Validate date using jquery

I have a date picker that enters the selected date inside the textbox. I need to make sure of the following before I save the date:

Make sure the field is not empty
Make sure the date is in YYYY-MM-DD format (ready to store in mysql date type column)
Make sure the date is at least 18yrs ago

				//Last Name
				var input= $("#last_name");
				var last_name = input.val();
				if(last_name != "" && validate_letters_only(last_name)){
					fd.append('last_name', last_name);
					input.removeClass("invalid").addClass("valid");
				}else{
					input.removeClass("valid").addClass("invalid");
				}
				
				// Birthday
				fd.append('birthdate', $("#birthday").val());

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Thanks in advance for your help