var DateValidator = dojo.declare("org.example.DefectDateValidate", null, {

    validate: function(attributeId, workItem, configuration) {
        var severity = "ERROR";
        var message = "your error msg here";

        var wi_type = workItem.getValue(WorkItemAttributes.TYPE);
        if (wi_type != "your work item type here") {
                // we may want to restrict the validation to a given type only
                return Status.OK_STATUS;
        }

        var beginDate = dojo.date.stamp.fromISOString(workItem.getValue("your begin date"));
        var endDate= dojo.date.stamp.fromISOString(workItem.getValue("your end date"));
               
        // Compare the two dates and make sure endDate is not earlier than beginDate
        if (dojo.date.compare(endDate, beginDate) >= 0) {
            return Status.OK_STATUS;
        }else if((beginDate == null) && (endDate == null)) {
            return Status.OK_STATUS;
        }else if((beginDate != null) && (endDate == null)) {
            return Status.OK_STATUS;
        }else {
            return new Status(Severity[severity], message);
        }
    }
});
})();

Hello Amit,
check the date validator sample here:
https://jazz.net/wiki/bin/view/Main/AttributeCustomization#Script_Based_Validation

Thanks.
Eric.