Issue of Invoice/Bill/Challan by a Service Tax assessee is mandatory as per Rule 4A of the STR, 1994. The same should be issued within 14 days from the date of completion of taxable service or receipt of payment towards the service, whichever is earlier. However, if the service is provided continuously for successive periods of time and the value of such taxable service is determined or payable periodically, the Invoice/Bill/Challan shall be issued within 14 days days of the date when each such event specified in the contract, which requires the service receiver to make any payment to service provider, is completed. ( Rule 4A (1) of the STR 1994).