In the case of an S corporation, net income shall be the amount (not below zero) determined by taking into account the aggregate amount of the S corporation’s items described in section
1366
(a) (other than credits and tax-exempt income). If the S corporation was a C corporation for the base year, its taxable income for such year shall be treated as its net income for such year (and such corporation shall be treated as an S corporation for such taxable year for purposes of paragraph (3)).