None of the rights and privileges herein granted and conveyed shall pas to the said Grantee unless said Grantee, within two years after the date when the Federal Power Commission shall authorize an allocation of natural gas to Grantee sufficient to serve the presently contemplated needs of the City, shall begin work laying its pipes to convey gas to the residents of the City and begins construction of its distribution systems within said City, and exercises due diligence in the completion thereof.