MVC custom validation: compare two dates

You need to modify your client-side script to check for the tested element’s prefix, and add the prefix (if any) to your selector, as follows:

$.validator.addMethod("isdateafter", function(value, element, params) {
    var parts = element.name.split(".");
    var prefix = "";
    if (parts.length > 1)
        prefix = parts[0] + ".";
    var startdatevalue = $('input[name="' + prefix + params.propertytested + '"]').val();
    if (!value || !startdatevalue) 
        return true;    
    return (params.allowequaldates) ? Date.parse(startdatevalue) <= Date.parse(value) :
        Date.parse(startdatevalue) < Date.parse(value);
});