(a) Except as otherwise provided in this section, the amount of tax imposed by this chapter shall be assessed within 3 years after the last day prescribed for filing the return or, if later, the date the return was filed.
(b) In the case of a false or fraudulent return with intent to evade tax or a failure to file a return, the tax may be assessed at any time.
(c) When, before the expiration of the time prescribed in subsection (a) of this section for the assessment of tax, both the Commissioner and the taxpayer have consented in writing to its assessment after such time, the taxpayer may be assessed at any time before the expiration of the period 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.