27. A pension, an annuity or a retirement allowance granted under the provisions of this act shall be effective only on the first day of a month, shall be paid in equal monthly installments, and shall not be decreased, increased, revoked or repealed, except as otherwise provided in this act; provided, however, that at the time any benefit becomes payable any unpaid balance of a loan or arrearage outstanding shall be deducted from any benefit otherwise payable.
Upon the death of a retirant, any unpaid benefits due him shall be paid in one lump sum to such person, if living, as he shall have nominated by written designation duly executed and filed with the board of trustees, otherwise to the executor or administrator of the retirant's estate. No pension, annuity or retirement allowance shall be due to a retirant or beneficiary unless it constitutes a payment for an entire month; provided, however, that a pension, annuity or retirement allowance shall be payable for the entire month in which the retirant or beneficiary dies.