California Code, Revenue and Taxation Code - RTC § 3726

A defense based on the alleged invalidity or irregularity of any proceeding instituted under this chapter can be maintained only in a proceeding commenced within one year after the date of execution of the tax collector's deed or within one year of the date the board of supervisors determines that a tax deed sold under this part should not be rescinded pursuant to Section 3731, whichever is later.