(b) Where, before the expiration of the period prescribed herein for the assessment of additional tax, a taxpayer has consented in writing that such period be extended, the amount of such additional tax due may be determined at any time within such extended period. The period so extended may be further extended by subsequent consents in writing made before the expiration of the extended period. If a taxpayer has consented in writing to the extension of the period for assessment, the period for filing an application for a refund pursuant to section fourteen hundred twelve shall not expire prior to six months after the expiration of the period within which an assessment may be made pursuant to the consent to extend the time for assessment of additional tax.