Any variance or modification of this chapter authorized by the Zoning Board of Appeals shall be automatically revoked unless a building permit conforming to all the conditions and requirements established by the Zoning Board of Appeals is obtained within nine months of the date of approval by the Zoning Board of Appeals and construction commenced within 15 months of such date of approval. A site plan approved by the Planning Board conforming to all of the conditions and requirements established by the Zoning Board of Appeals shall be obtained within 12 months of the date of the approval by the Zoning Board of Appeals and construction commenced within 18 months of such date of approval.