26 C.F.R. § 1.410(a)-8
Sections 410(a)(5)(D) and 411(a)(6)(D), as amended by the Retirement Equity Act of 1984 (REA 1984), permit a plan to disregard years of service that were disregarded under the plan provisions satisfying those sections (as in effect on August 22, 1984) as of the day before the REA amendments apply to the plan. Under section 302(a) of REA 1984, the new break-in-service rules generally apply to plan years beginning after December 31, 1984. Thus, for example, assume a plan has a calendar plan year and disregarded years of service as permitted by sections 410(a)(5)(D) and 411(a)(6)(D) as in effect on August 22, 1984. An employee completed two years of service in 1981 and 1982, and then incurred two consecutive 1-year breaks in service in 1983 and 1984. The plans may disregard the prior years of service even though the employee did not incur five consecutive 1-year breaks in service. On the other hand, assume the employee completed three consecutive years of service beginning in 1980, and incurred two 1-year breaks in service in 1983 and 1984. Because, as of December 31, 1984, the years of service credited before 1983 could not be disregarded, whether the plan may subsequently disregard those years of service would be governed by the rules enacted by REA 1984.
26 C.F.R. §1.410(a)-8