contributions are made on behalf of other employees eligible to participate in the arrangement, and (II) an employer may elect to exclude such employees from the application of subsection (a)(4), paragraph (3), subsection (m)(2), and section 410(b). (ii) Top-heavy rules An employer may elect to exclude all employees who are eligible to participate in a plan maintained by the employer solely by reason of paragraph (2)(D)(ii) from the application of the vesting and benefit requirements under subsections (b) and (c) of section 416. (iii) Vesting For purposes of determining whether an employee described in clause (i) has a nonforfeitable right to employer contributions (other than contributions described in paragraph (3)(D)(i)) under the arrangement, each 12-month period for which the employee has at least 500 hours of service shall be treated as a year of service. (iv) Employees who become full-time employees This subparagraph shall cease to apply to any employee as of the first plan year beginning after the plan year in which the employee meets the requirements of section 410(a)(1)(A)(ii) without regard to paragraph (2)(D)(ii) of this subsection. (C) Exception for employees under collectively bargained plans, etc Paragraph (2)(D)(ii) shall not apply to employees described in section 410(b)(3). (D) Special rules (i) Time of participation The rules of section 410(a)(4) shall apply to an employee eligible to participate in an arrangement solely by reason of paragraph (2)(D)(ii). (ii) 12-month periods 12-month periods shall be determined in the same manner as under the last sentence of section 410(a)(3)(A). . (b) Effective date The amendments made by this section shall apply to plan years beginning after December 31, 2018, except that, for purposes of section 401(k)(2)(D)(ii) of the Internal Revenue Code of 1986 (as added by such amendments), 12-month periods beginning before January 1, 2019, shall not be taken into account. 106. Separate application of top heavy rules to defined contribution plans covering part-time