IL-1120 returns are due on different dates depending on the type of corporation and the tax year end date:

• If the tax year ends on a date other than June 30, Form IL-1120 is due on or before the 15th day of the 4th month following the close of the tax year. There is an automatic six month extension of time to file the return.

• If the tax year ends on June 30, Form IL-1120 is due on or before the 15th day of the 3rd month following the close of the tax year. There is an automatic seven month extension of time to file the return.

• If the corporation is a cooperative, Form IL-1120 is due on the 15th day of the 9th month following the close of the tax year regardless of when the tax year ends. There is an automatic seven month extension of time to file the return.

IL-1120-ST returns are due the 15th day of the third month following close of the tax year. There is an automatic seven month extension of time to file the return.

For both IL corporate and S corp returns, the following applies:

• No extension form is required to be submitted to extend the due date for filing the return.

• Payment of the total tax must be made by the original due date. The payment date cannot be extended.

  • Form IL-1120-V (for corporate) or Form IL-1120-ST-V (for S corp) should be used to submit any expected tax due.

See the IL-1120 Instructions and the IL-1120-ST Instructions for details.