A » The deadline for filing corporate tax returns in the United States depends primarily on the corporation's tax year structure, with the most common rule being that for a calendar-year C corporation (which follows the January 1 to December 31 year), Form 1120 must be filed by the 15th day of the fourth month after the close of the tax year—that is, by April 15. However, if a corporation uses a fiscal year that ends on a different date, the return is due by the 15th day of the fourth month following the end of that fiscal year. For example, a fiscal year ending June 30 would have a deadline of October 15. It is critical to note that S corporations, which file Form 1120-S, operate under a different schedule: for a calendar-year S corp, the deadline is March 15 (the 15th day of the third month after year-end), and for a fiscal-year S corp, the deadline is the 15th day of the third month after the close of the fiscal year. Furthermore, corporations can request an automatic six-month extension by filing Form 7004, which moves the filing deadline to October 15 for calendar-year C corporations (and to September 15 for calendar-year S corporations). It is important to understand that an extension to file is not an extension to pay; any taxes owed must still be paid by the original due date to avoid penalties and interest. Additionally, corporations generally must make estimated tax payments throughout the year, with deadlines typically falling on April 15, June 15, September 15, and December 15 for calendar-year taxpayers, though the fourth-quarter payment for C corporations is due by the 15th day of the 12th month (December 15) rather than the 15th day of the first month of the next year. For corporations that operate on a fiscal year, the estimated payment schedule is adjusted accordingly. These deadlines are governed by Internal Revenue Code Sections 6072 and 6655, and they apply to domestic corporations; foreign corporations with a U.S. trade or business also follow similar rules but may have additional requirements under Section 6072(c). There are also special rules for tax-exempt organizations filing Form 990-T for unrelated business income, and for corporations in disaster areas, where deadlines may be extended. Given the complexity and potential for severe penalties for late filing (typically 5% per month of the unpaid tax, up to 25%) and late payment (0.5% per month), corporations should consult with a qualified tax professional to ensure compliance with all applicable deadlines and to take advantage of any available extensions or relief provisions. The deadlines highlighted here are for federal purposes; state corporate tax deadlines can vary significantly and must be verified separately, as some states align with federal dates while others impose earlier or later filing dates. Ultimately, the precise deadline is determined by the corporation's specific tax year and classification, making careful record-keeping and timely filings essential to avoid costly noncompliance.