14 U.S.C. § 3747: Date of rank upon promotion; entitlement to pay

When a Reserve officer is promoted to the next higher grade under this subchapter, the date of rank shall be the date of appointment in that grade, unless the promotion was determined in accordance with a running mate system, in which event the same date of rank shall be assigned as that assigned to the officer’s running mate. A Reserve officer so promoted shall be allowed the pay and allowances of the higher grade for duty performed from the date of the officer’s appointment thereto. Notwithstanding any other provision of law and subject to subsection (c), if promotion of an inactive duty promotion list officer to the grade of rear admiral or rear admiral (lower half) is determined in accordance with a running mate system, a reserve officer, if acceptable to the President and the Senate, shall be promoted to the next higher grade no later than the date the officer’s running mate is promoted. if a delay in the finding required under section 3745(a) of this title is beyond the control of the officer and the officer is otherwise qualified for promotion; or for any other reason that equity requires.