For the purposes of this subchapter, the period of service of a participant shall be computed from the date on which the participant becomes a participant under this subchapter.
In computing the period of service of a participant, all periods of separation from the Agency and so much of any leave of absence without pay as may exceed six months in the aggregate in any calendar year shall be excluded, except leaves of absence while receiving benefits under chapter 81 of title 5 and leaves of absence granted participants while performing active and honorable service in the Armed Forces.
A participant or former participant who returns to Government duty after a period of separation shall have included in the participant or former participant’s period of service that part of the period of separation in which the participant or former participant was receiving benefits under chapter 81 of title 5.
Each year of duty at a post on the list established under paragraph (1), inclusive of regular leaves of absence, shall be counted as one and a half years in computing the length of service of a participant under this subchapter for the purpose of retirement. In computing such service, any fractional month shall be treated as a full month.