(a) Except in the case of fraud or failure to file a return required by this title, every notice of a deficiency assessment or the issuance of an execution thereon shall be given within seven years from the date on which such return is filed. In the case of failure to file a return, the notice of a deficiency assessment or the issuance of an execution thereon shall be given within ten years from the date on which such return is due. In the case of fraud there shall be no time limitation.

(b) If, before the expiration of the time prescribed in this Code section for giving of a notice of deficiency assessment or before the issuance of an execution thereon, the taxpayer has consented in writing to the giving of the notice after such time, the notice may be given at any time prior to the expiration of the time agreed upon. The period so agreed upon may be extended by subsequent agreements in writing made before the expiration of the period previously agreed upon.