Article 17 opens Chapter 7 of the Decree-Law, which deals with the Tax Period, the Tax Return and the payment of Tax. The Article does not itself fix the length of the Tax Period. It provides instead that the Executive Regulation of the Decree-Law shall specify the Tax Period, and shall also specify the exceptional circumstances under which the Authority may amend the Tax Period. Both the duration of the period for which the Payable Tax is calculated and the grounds on which the Authority may vary it are therefore left to the Executive Regulation rather than stated in the Decree-Law.