Section 11-2050. Transitional provisions

The  taxes  imposed  by  this
  subchapter shall be paid with respect to  receipts  from  all  sales  of
  services  on  or after September first, nineteen hundred eighty although
  rendered or agreed to be  rendered  under  a  prior  contract.  Where  a
  service is sold on a monthly, quarterly, yearly or other term basis, the
  charge for such service shall be subject to tax under this subchapter to
  the  extent  that  such  charge  is applicable to any period on or after
  September first, nineteen hundred  eighty,  and  such  charge  shall  be
  apportioned  on  the  basis  of  the ratio of the number of days falling
  within such period to the total number of  days  in  the  full  term  or
  period.