(a) General rule. For the purpose of determining the days of service which must be credited to an employee for a computation period, a plan shall determine days of service from records of days worked and days for which payment is made or due. Any records may be used to determine days of service to be credited to employees under a plan, even though such records are maintained for other purposes, provided that they accurately reflect the actual number of days of service with which an employee is required to be credited under § 2530.200b-7(a). Payroll records, for example, may provide sufficiently accurate data to serve as a basis for determining days of service. If, however, existing records do not accurately reflect the actual number of days of service with which an employee is entitled to be credited, a plan must develop and maintain adequate records. A plan may in any case credit days of service under any method which results in the crediting of no less than the actual number of days of service required to be credited under § 2530.200b-7(a) to each employee in a computation period, even though such method may result in the crediting of days of service in excess of the number of days required to be credited under § 2530.200b-7(a). A plan is not required to prescribe in its documents which records are to be used to determine days of service.

(b) Determination of pre-effective date days of service. To the extent that a plan is required to determine days of service completed before the effective date of part 2 of title I of the Act (see section 211 of the Act), the plan may use whatever records may be reasonably accessible to it and may make whatever calculations are necessary to determine the approximate number of hours of service completed before such effective date. For example, if a plan or an employer maintaining the plan has, or has access to, only the records of compensation of employees for the period before the effective date, it may derive the pre-effective date days of service by using the daily rate for the period or the days customarily worked. If accessible records are insufficient to make an approximation of the number of pre-effective date days of service for a particular employee or group of employees, the plan may make a reasonable estimate of the days of service completed by such employee or employees during the particular period. For example, if records are available with respect to some employees, the plan may estimate the days of service of other employees in the same job classification based on these records. A plan may use the elapsed time method prescribed under § 2530.200b-9 to determine days of service completed before the effective date of part 2 of title I of the Act.