I believe it would be due September 15. For the first year, you file a short-year return for May 11-June 30th.From the instructions for Form 1120:Generally, a corporation must file its income tax return by the 15th day of the 4th month after the end of its tax year. A new corporation filing a short-period return must generally file by the 15th day of the 4th month after the short period ends. A corporation that has dissolved must generally file by the 15th day of the 4th month after the date it dissolved.However, a corporation with a fiscal tax year ending June 30 must file by the 15th day of the 3rd month after the end of its tax year. A corporation with a short tax year ending anytime in June will be treated as if the short year ended on June 30, and must file by the 15th day of the 3rd month after the end of its tax year.*Disclaimer: I am just providing some general information. Please consult a tax professional about your personal situation.