A member of the state employees retirement system who is or has been granted a leave of absence without pay for the protection or improvement of his health shall be entitled to credit toward retirement for the period of such leave, but not to exceed twelve months within a five-year period, if he makes retirement contributions for each month of such leave in an amount equal to one-twelfth of five per cent of his salary rate at the time he was granted leave, with interest at the rate of five per cent per year from the date when any such contributions would have been due to the date of payment.