Form 1065-B (electing large partnerships).   This form is due on the 15th day of the 4th month after the end of the partnership's tax year. Provide each partner with a copy of Schedule K-1 (Form 1065-B) or a substitute Schedule K-1 by the first March 15 following the close of the partnership's tax year.

Form 2553.   This form is used to choose S corporation treatment. It is due no more than two months and 15 days after the beginning of the tax year the election is to take effect or at any time during the preceding tax year.