A. If an individual has filed a separate return for a taxable year for which the individual and spouse could have filed a joint return under section 43-309, and the time prescribed by this title for filing the return for such taxable year has expired, such individual and spouse may nevertheless make a joint return for such taxable year. A joint return filed by the husband and wife for such taxable year, and all payments, credits, refunds or other repayments made or allowed with respect to the separate return of either spouse for such taxable year, shall be taken into account in determining the extent to which the tax based upon the joint return has been paid.
B. A joint return may be made under subsection A of this section only if there is paid in full at or before the time of the filing of the joint return:
1. All amounts previously assessed with respect to either spouse for such taxable year.
2. All amounts shown as the tax by either spouse upon his or her separate return for such taxable year.
3. Any amount determined, at the time of the filing of the joint return, as a proposed deficiency with respect to either spouse for such taxable year if, prior to such filing, a notice under section 42-1108 of such proposed deficiency has been mailed.
C. A joint return may not be made under subsection A of this section:
1. After the expiration of four years from the last date prescribed by law for filing the return for such taxable year, determined without regard to any extension of time granted to either spouse.
2. After there has been mailed to either spouse, with respect to such taxable year, a notice of deficiency under section 42-1108, if the spouse, as to such notice, appeals to the department under section 42-1251, or appeals to the state board under section 42-1253.
3. After either spouse has commenced a suit in any court for the recovery of any part of the tax for such taxable year.
4. After either spouse has entered into a closing agreement under section 42-1113, with respect to such taxable year.
D. If a joint return is made under subsection A of this section, any election, other than the election to file a separate return, made by either spouse in the separate return for such taxable year with respect to the treatment of any income, deduction or credit of such spouse shall not be changed in the making of the joint return where such election would have been irrevocable if the joint return had not been made.
E. If a joint return is made under subsection A of this section after the death of either spouse, such return with respect to the decedent may be made only by the decedent's personal representative.
F. Where the amount shown as the tax by the husband and wife on a joint return made under subsection A of this section exceeds the aggregate of the amounts shown as the tax upon the separate return of each spouse:
1. If any part of such excess is attributable to negligence or intentional disregard of rules and regulations, but without intent to defraud, at the time of the making of such separate return, then five per cent of the total amount of such excess shall be assessed, collected and paid as if it were a deficiency.
2. If any part of such excess is attributable to fraud with intent to evade tax at the time of the making of such separate return, then fifty per cent of the total amount of such excess shall be so assessed, collected and paid in lieu of the fifty per cent addition to the tax provided in section 42-1125.
G. For the purposes of section 42-1104, relating to periods of limitations upon assessment and collection, and for the purposes of section 42-1125, subsection A, relating to delinquent returns, a joint return made under this section shall be deemed to have been filed:
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 title 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 less than one thousand dollars of taxable income and less than five thousand dollars of gross income for such taxable year, on the date of the filing of such separate return, but not earlier than the last date prescribed by law for the filing of such separate return.
3. Where only one spouse filed a separate return prior to the making of a joint return and the other spouse had a taxable income of more than one thousand dollars or a gross income of more than five thousand dollars for such taxable year, on the date of the filing of such joint return.
H. For the purposes of section 42-1118, relating to refunds and credits, a joint return made under this section shall be deemed to have been filed on the last date prescribed by this title for filing the return for such taxable year, determined without regard to any extension of time granted to either spouse.
I. If a joint return is made under subsection A of this section, the period of limitations provided in section 42-1104 on the making of assessments and collecting taxes shall with respect to such return include one year immediately after the date of the filing of such joint return, computed without regard to the provisions of subsection G of this section.
J. For the purposes of section 42-1125, relating to penalties in the case of fraudulent returns, the term "return" includes a separate return filed by a spouse with respect to a taxable year for which a joint return is made under subsection A of this section after the filing of such separate return.