5 CFR § 351.506 - Effective date of retention standing.
Except for applying the performance factor as provided in § 351.504:
(b) The retention standing of each employee retained in a competitive level as an exception under § 351.606(b), § 351.607, or § 351.608, is determined as of the date the employee would have been released had the exception not been used. The retention standing of each employee retained under any of these provisions remains fixed until completion of the reduction in force action which resulted in the temporary retention.
(c) When an agency discovers an error in the determination of an employee's retention standing, it shall correct the error and adjust any erroneous reduction-in-force action to accord with the employee's proper retention standing as of the effective date established by this section.