Section 13-307. Extra service pension credits

Except  as  provided  in
  subdivisions a and b of section 13-304 of this chapter, a member who has
  served the minimum period of time elected by him or her  for  retirement
  may  continue  in  the  service.  In  such  event  and  upon  his or her
  retirement for any cause whatsoever, there shall be added to his or  her
  annual  pension  to  which he or she shall upon his or her retirement be
  entitled an additional amount computed at the rate  of  one-sixtieth  of
  his  or  her  final salary for each year of such additional service. Any
  such member who has elected to contribute on  the  basis  of  retirement
  after  twenty  years  of  service  and  who  thereafter continues in the
  service shall have his or her deductions made at the rate  of  five  per
  cent per annum after completing twenty-five years of service.