onValidation.js

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

var cEnd = ProcessHandlingUtils.getOnValidationValue(vars.get("$field.ENDDATE"));

if (DateUtils.validateBeginnBeforeEnd(vars.get("$field.STARTDATE"), cEnd) === false)
    result.string(DateUtils.getValidationFailString());