Rev. & Tax. Code, § 18528

(a) For the purposes of Sections 19057 to 19067, inclusive (relating to period of limitations upon assessment and collection), and for the purposes of Section 19131 (relating to delinquent returns), a joint return made under Section 18522 shall be deemed to have been filed as follows:

(1) Where both spouses filed separate returns prior to making the joint return, on the date the last separate return was filed (but not earlier than the last date prescribed by this part for filing the return of either spouse).

(2) Where one spouse filed a separate return prior to the making of the joint return, and the other spouse had eight thousand dollars ($8,000) or less of adjusted gross income from all sources and ten thousand dollars ($10,000) or less of gross income from all sources for the taxable year, on the date of the filing of the separate return (but not earlier than the last date prescribed by this part for the filing of the separate return).

(3) Where only one spouse filed a separate return prior to the making of a joint return and the other spouse had an adjusted gross income from all sources in excess of eight thousand dollars ($8,000) or a gross income from all sources in excess of ten thousand dollars ($10,000) for the taxable year, on the date of the filing of the joint return.

(b) For purposes of Article 1 (commencing with Section 19301) of Chapter 6, a joint return made under Section 18522 shall be deemed to have been filed on the later of the last date prescribed by this part for filing the return for the taxable year (determined without regard to any extension of time granted to either spouse) or the date the later timely filed separate return was filed.