The tax imposed under this title shall be paid on the fifteenth day of April following the close of the calendar year or, if the return is made on the basis of a fiscal year, on the fifteenth day of the fourth month following the close of the fiscal year, except that:

1. 1. For an S corporation the tax imposed shall be paid on the fifteenth day of the third month following the close of the taxable year.
2. 2. For unrelated business taxable income of a tax exempt organization the tax imposed shall be paid on the fifteenth day of the fifth month following the close of the taxable year.