onValidation.js

import("system.neon");
import("system.result");
import("system.vars");
import("Date_lib");

var dateUtils = new DateUtils();
var cStart = vars.get("$local.value");
var cDue = vars.get("$field.CONTRACTDUE");

if (dateUtils.validateBeginnBeforeEnd(cStart, vars.get("$field.CONTRACTEND")) === false)
    result.string(dateUtils.getValidationFailString());