13. Where a retirement pension becomes payable to a contributor on a date other than that of his 65th birthday, the basic monthly amount thereof shall be adjusted as follows:(1) where that date falls before the date of the contributor’s 65th birthday, the basic monthly amount shall be decreased by 0.5% for each month between the date on which the pension becomes payable to him and the date of his 65th birthday; or
(2) where that date falls after the date of the contributor’s 65th birthday, the basic monthly amount shall be increased by 0.5% for each month, up to a limit of 60 months, between the date of his 65th birthday and the date on which the pension becomes payable to him.