September 16 -Â Corporations File a 2012 calendar year income tax return (Form 1120 or 1120-A) and pay any tax, interest, and penalties due. This due date applies only if you timely requested an automatic 6-month extension. September 16 - S Corporations File a 2012 calendar year income tax return (Form 1120S) and pay any tax due. This due date applies only if you requested an … [Read more...] about September 2013 Business Due Dates