1. An individual who is a resident for only part of his taxable period shall be treated as a nonresident for purposes of sections 143.011 to 143.996. His Missouri nonresident adjusted gross income (Missouri adjusted gross income from sources within this state) shall consist of
(1) All items that would have determined his Missouri adjusted gross income if he had a taxable period as a resident consisting solely of the time he was a resident, and
(2) All items that would have determined his Missouri nonresident adjusted gross income if he had a taxable period as a nonresident consisting solely of the time he was not a resident.
2. An individual described in subsection 1 may determine his tax as if he were a resident for the entire taxable period.