Greetings

You have asked for advice on the deadline for making an election with the IRS to be taxed as an "S corporation". If you have not done so already I recommend that you consult with a tax professional to answer that question and to help you determine if making such an election would be your best course of action.

The election to be taxed as an S Corporation is made by filing IRS IRS form 2553. 

You can download IRS form 2553 and the instructions from the IRS website. To do so to IRS.GOV and then enter "Form 2553" into the search bar in the upper right hand corner of the page.  Doing that will not only give a link to download the form itself but also a link to the instructions and helpful information about S Corporations.  Read the instructions to find out about the filing deadline.

Most corporations use the calendar year as their tax year.  The election to be taxed as an S corporation normally must be made within two months after the end of tax year for which the status is sought.  It's a bit more complicated than that.  You should carefully read the instructions to Form 2553 and consult with a tax professional.