5 C.F.R. § 843.503 – Commencing and terminating dates.

(a) An annuity under this subpart commences on the day after the retiree dies.
(b) An annuity under this subpart terminates on the last day of the month before the insurable interest beneficiary dies.
(c) A survivor annuity accrues on a daily basis, one-thirtieth of the monthly rate constituting the daily rate. An annuity does not accrue for the 31st day of any month, except in the initial month if the survivor's (of a deceased employee) annuity commences on the 31st day. For accrual purposes, the last day of a 28-day month constitutes 3 days and the last day of a 29-day month constitutes 2 days.