If the variance or special exception is granted or the issuance of a permit is approved, or other action by the appellant is authorized, the necessary permit shall be secured and the authorized action begun within one year after the date when the variance or special exception is finally granted, or the issuance of a permit is finally approved, or the other action by the appellant is authorized, and the building or alteration, as the case may be, shall be completed within two years of said date. For good cause, the Zoning Hearing Board may, at any time, upon application in writing, extend either of these deadlines.