JSFiddle - React, Tailwind, and code Playground

HTML

<link rel="stylesheet" href="http://ajax.googleapis.com/ajax/libs/jqueryui/1.8/themes/dot-luv/jquery-ui.css">
<script src="http://ajax.microsoft.com/ajax/jquery.validate/1.7/jquery.validate.js"></script>
<script src="http://ajax.microsoft.com/ajax/jquery.validate/1.7/localization/methods_de.js"></script>
<form>
    <input type="text" name="dateField" /><br />
    <input type="Submit" value="Test" />
</form>

CSS

.error { color: red; }

JavaScript

$.validator.addMethod("dateISODE", function(value, element) { 
    return $.validator.methods.dateISO.apply(this, arguments) 
        || $.validator.methods.date.apply(this, arguments); 
}, "Please enter a valid ISO or German date");

$("form").validate({
    rules: {
        dateField: {
            required: true,
            dateISODE: true
        }
    },
    debug: true
});