Generally, an S corporation must file Form 1120S by the 15th day of the 3rd month after the end of its tax year. For calendar year corporations, the due date is March 15.
A corporation that has dissolved must generally file by the 15th day of the 3rd month after the date it dissolved.
The corporation can file on the next day that is not a Saturday, Sunday, or legal holiday.
If the S corporation election was terminated during the tax year and the corporation reverts to a C corporation, file Form 1120S for the S corporation's short year by the due date (including extensions) of the C corporation's short year return.
File Form 7004 Application for Automatic Extension of Time To File Certain Business Income Tax, Information, and Other Returns, to request a 6-month extension of time to file.
Part I includes, in form number order, all the forms for which Form 7004 is used to request an extension of time to file. Filers requesting an extension will enter (in the box located at the top of Part I) the form code for the return for which the extension is requested. Part II includes questions for all filers.