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("oneOf", function(value, element, params) {
    for(p in params) {
        if(params.hasOwnProperty(p) && $.validator.methods[p].apply(this, [value, element, params[p]])) {
            return true;
        }
    };
    return false;
}, "Please enter a valid ISO or German date");

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