C-Corporations follow a hard deadline of Wednesday, April 15, 2026. Taxzerone Corporations with a short tax year that began before January 1, 2026, and ends on June 30, 2026, must file by September 15, 2026. Form 7004 is used to request an automatic 6-month extension of time to file Form 1120. Internal Revenue Service Consequently, the extended deadline for calendar-year filers using Form 7004 is October 15, 2026 — but that extension covers paperwork only, not payment.