For the purposes of this article, each of the following shall apply:

1. (a) The status as spouses of two individuals having taxable years beginning on the same day shall be determined as follows:

   1. (1) If both have the same taxable year, then as of the close of that year.
   2. (2) If one dies before the close of the taxable year of the other, then as of the time of the death.
2. (b) An individual who is legally separated from his or her spouse under a decree of divorce, termination of registered domestic partnership, or of separate maintenance shall not be considered as married or in a registered domestic partnership.
3. (c) If a joint return is made, the tax shall be computed on the aggregate income and the liability with respect to the tax shall be joint and several.