Treasury Regulation § 1.401(a)(4)-11(d)(3)(iii) addresses the rules for granting such pre-participation service:
(iii)Requirements for pre-participation and imputed service -
(A)Provision applied to all similarly-situated employees -
(1)General rule. A plan provision crediting pre-participation service or imputed service to any HCE must apply on the same terms to all similarly-situated NHCEs. Whether two employees are similarly situated for this purpose must be determined based on reasonable business criteria, generally taking into account only the circumstances resulting in the employees being covered under the plan or being granted imputed service and on the situation of the employees (e.g., the plan in which the employees benefit or the employer by which they are employed) during the period for which the pre-participation service or imputed service is credited. For example, employees who enter a plan as a result of a particular merger and who participated in the same plan of a prior employer are generally similarly situated. As another example, employees who are transferred to different joint ventures or different spun-off divisions are generally not similarly situated.
(2)Examples. The following examples illustrate the rules in this paragraph (d)(3)(iii)(A) . . .