Section 11-2041. Transitional provisions

The taxes imposed under paragraph
  one of subdivision (a) of section 11-2040 of this  subchapter  shall  be
  paid  with  respect  to  receipts from all sales of services on or after
  September first, nineteen  hundred  seventy-five  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  seventy-five, 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.