Sections 838.242 and 838.441 contain information on how OPM calculates lengths of service.

Unless the court order otherwise expressly directs—

For the purpose of describing a period of time to be excluded from any element of a computation, the term “military service” means military service as defined in section 8331(13) of title 5, United States Code, and does not include civilian service with the Department of Defense or the Coast Guard; and

For the purpose of describing a period of time to be included in any element of a computation, the term “military service” means all periods of military and civilian service performed with the Department of Defense or the Coast Guard.

When a court order directed at employee annuity (other than a phased retirement annuity or a composite retirement annuity) contains a formula for dividing employee annuity that requires a computation of service worked as of a date prior to separation and using terms such as “years of service,” “total service,” “service performed,” or similar terms, the time attributable to unused sick leave will not be included.

When a court order directed at employee annuity other than a phased retirement annuity or a composite retirement annuity contains a formula for dividing employee annuity that requires a computation of “creditable service” (or some other phrase using “credit” or its equivalent) as of a date prior to retirement, unused sick leave will be included in the computation as follows:

If the amount of unused sick leave is specified, the court order awards a portion of the employee annuity equal to the monthly employee annuity at retirement times a fraction, the numerator of which is the number of months of “creditable service” as of the date specified plus the number of months of unused sick leave specified (which sum is rounded to eliminate partial months) and whose denominator is the months of “creditable service” used in the retirement computation.

If the amount of unused sick leave is not specified, the court order awards a portion of the employee annuity equal to the monthly rate at the time of retirement times a fraction, the numerator of which is the number of months of “creditable service” as of the date specified (no sick leave included) and whose denominator is the number of months of “creditable service” used in the retirement computation (sick leave included).

General language such as “benefits earned as an employee with the U.S. Postal Service * * *” provides only that CSRS or FERS retirement benefits are subject to division and does not limit the period of service included in the computation (i.e., service performed with other Government agencies will be included).

To limit the computation of benefits other than a phased retirement annuity or a composite retirement annuity to a particular period of employment, the court order must—

Use language expressly limiting the period of service to be included in the computation (e.g., “only U.S. Postal Service” or “exclusive of any service other than U.S. Postal Service employment”); or

Specify the number of months to be included in the computation; or

Describe specifically the period of service to be included in the computation (e.g., “only service performed during the period Petitioner and Defendant were married” or “benefits based on service performed through the date of divorce”).

A court order directed at a phased retirement annuity or a composite retirement annuity cannot limit the computation and division of a phased retirement annuity or composite retirement annuity to a particular period of employment or service. A phased retirement annuity is based on an employee's service as of phased retirement and a “fully retired phased component,” described in §§ 831.1742 and 848.502, of a composite retirement annuity is based on a phased retiree's service as of his or her full retirement. A court order that attempts to limit the computation of a phased retirement annuity or a composite retirement annuity to a particular period of employment or service is not a court order acceptable for processing. If the former spouse's award of a portion of phased retirement annuity or a composite retirement annuity is to be limited, the limitation of the division must be accomplished in a manner other than by limiting the service to be used in the computation.