March 15 – Corporations.  File a 2003 calendar year income tax return (Form 1120 or 1120–A) and pay any tax due.  If you want an automatic 6-month extension of time to file the return, file Form 7004 and deposit what you estimate you owe.

March 15 – S corporations.  File a 2005 calendar year income tax return (Form 1120S) and pay any tax due.  Provide each shareholder with a copy of Schedule K–1 (Form 1120S), Shareholder's Share of Income, Credits, Deductions, etc., or a substitute Schedule K–1.  If you want an automatic 6-month extension of time to file the return, file Form 7004 and deposit what you estimate you owe.

March 15 – S corporation election.  File Form 2553, Election by a Small Business Corporation, to choose to be treated as an S corporation beginning with calendar year 2006.  If Form 2553 is filed late, S treatment will begin with calendar year 2005.

March 15 – Electing large partnerships.  Provide each partner with a copy of Schedule K–1 (Form 1065–B), Partner's Share of Income (Loss) From an Electing Large Partnership, or a substitute Schedule K–1.  This due date is effective for the first March 15 following the close of the partnership's tax year.  The due date of March 15 applies even if the partnership requests an extension of time to file the Form 1065–B by filing Form 8736 or Form 8800.

March 31 – Electronic filing of Forms 1098, 1099, and W–2G.  File Forms 1098, 1099, or W–2G with the IRS.  This due date applies only if you file electronically (not by magnetic media).  Otherwise, see March 1.

The due date for giving the recipient these forms still will be February 2.

For information about filing Forms 1098, 1099, or W–2G electronically, see Publication 1220, Specifications for Filing Forms 1098, 1099, 5498 and W–2G Electronically or Magnetically.

Note.  The 3 months that make up each quarter of a fiscal year may be different from those of each calendar quarter, depending on when the fiscal year begins.

Partnerships.  Form 1065.  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) or a substitute Schedule K–1.

Partnerships.  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) or a substitute Schedule K–1.

Corporations and S Corporations.  Form 1120 and Form 1120S (or Form 7004).  These forms are due on the 15th day of the 3rd month after the end of the corporation's tax year.  S Corporations must provide each shareholder with a copy of Schedule K–1 (Form 1120S) or a substitute Schedule K–1.

Estimated tax payments.  Payments are due on the 15th day of the 4th, 6th, 9th, and 12th months of the corporation's tax year.

Form 2553.  This form is used to choose S corporation treatment.  It is due by the 15th day of the 3rd month of the first tax year to which the choice will apply or at any time during the preceding tax year.