A deduction from gross income shall be allowed to those cellular telephone service companies which keep their regular books of account on an accrual basis, for credit losses, actually sustained by such a taxpayer as a result of its cellular telephone service business. Twenty percent of such credit losses shall be allowed as a deduction during the calendar year 1995; forty percent of such credit losses shall be allowed as a deduction during calendar year 1996; sixty percent of such credit losses shall be allowed as a deduction during calendar year 1997; eighty percent of such credit losses shall be allowed as a deduction during calendar year 1998 and all such credit losses shall be allowed as a deduction for calendar years 1999 and thereafter. (Ord. 1009 §4, 1994).