Today marks the deadline for filing extensions for pass through entities, including S Corporations (1120S) and Partnerships (1065). If your business uses a fiscal year, you generally need to file your tax return by the 15th day of the third month following the close of your tax year. For example, if your business uses an April 1 - March 31 tax year, your business tax return would be due June 15 instead of March 15.