As long as you havenu2019t rescinded a prior election to be taxed as an S Corporation vs. as a C Corporation, you may make the election at any time. However, you cannot go back more than one year.Technically, you must make the election by the fifteenth day of the third month of the calendar year unless you chose a fiscal year end (which is generally not an available option, although there are exception available under Section 444 of the IRS Code), which means not later than March 15 of the year in which you wish to begin reporting income as an S Corporation.There is, however, what is called u201clate election reliefu201d available, by invoking Revenue Procedure 2013u201330. You may therefore make the election and then file it along with the S Corporation Tax Return.However, there may be problems, if you extended your return. In that event, the IRS will not be aware that you made the election and, even if you filed a Form 7004 to extend the newly elected S Corporation, they will not recognize the extension and charge a late-filing penaltyu2026at first. However, when you do file the return that was previously on extension including the Form 2553 S Corp election and invoke the late election relief provision, simply attach a copy of the Form 7004. The Service may still send a notice of late filing, but you then simply have to explain that you extended the return on or before March 15, attach the proof of having filed the extension (either a proof of having e-filed the extension or a copy of the paper form, including proof of mailing in the form of a Certified Mail Receipt - if you ever mail anything to the IRS, always use a Certified Mail Receipt!), and the IRS will abate the penalty.