“(3A) Where an employer agrees in writing with an employee to alter the date of dismissal mentioned in a notice under subsection (1) (a) given by him to that employee so as to ensurethat the employee's notice under subsection (1) (b) will be within the obligatory period in relation to the notice under subsection (1) (a), the employee's entitlement to redundancy payment shall be unaffected and the employee shall, for the purposes of this Part, be taken to be dismissed by his employer, the date of dismissal in relation to that dismissal being the date on which the employee's notice expires.”.