The September 15 Business Tax Deadline

S corps and partnerships on extension file that Tuesday. Calendar year C corps do not. Here is the full list of dates, what a late return costs per owner, and what to do if the books are not ready.

What is actually due that Tuesday

September 15, 2026 is the extended federal deadline for exactly two business returns. Form 1065, filed by partnerships and by multi member LLCs that did not elect corporate treatment. And Form 1120-S, filed by S corporations. Both were originally due March 16, 2026, and filing Form 7004 bought six more months. That extension is now spent.

If you are a sole proprietor, a single member LLC, or anyone filing a Schedule C on your personal return, nothing is due from you on September 15 on the return side. Your Form 1040 on extension runs to October 15.

The C corporation everybody miscounts

This is the one I correct most often. People hear “the corporate deadline” and assume every corporation files the same day. A calendar year C corporation filing Form 1120 was due April 15, 2026, and the automatic extension is a full six months, which lands on October 15, 2026.

The confusion is not anyone’s fault. For years the code carried a quirk that shortened the extension for December year end C corps, and half the internet still repeats it. The current Form 7004 instructions are plain about it: the automatic extension period is generally six months, with exceptions only for certain estates and trusts and for C corporations with a June 30 year end.

There is one C corp that genuinely does file on September 15, and it is the fiscal year filer with a November 30 year end. Original due date March 15, six months out, September 15. If your year closes in November, this article is about you after all.

California runs a different clock

Here is where San Diego owners talk themselves into trouble. California does not make you file an extension form at all. If your entity is in good standing, the extension is automatic and paperless. But the length is not the same as the federal one.

• California S corporation, Form 100S: six month automatic extension, so September 15, 2026. Same day as federal.
• California partnership, Form 565, and LLC, Form 568: seven months, so October 15, 2026.

So if you run an LLC taxed as a partnership, your federal 1065 is due September 15 and your state return is not due until October 15. That extra month is real, and it is also exactly how people convince themselves they have more time than they do. Your K-1s come off the federal return. Do not wait on it.

One more thing while you are in there. The $800 minimum franchise tax and the LLC fee came due earlier in the year, and the extension never covered either one. An extension to file has never been an extension to pay.

What missing it actually costs

The late filing penalty on a 1065 or an 1120-S is not a percentage of tax owed. It is a flat charge per owner, per month. For 2025 returns it runs $255 per partner or shareholder for each month or part of a month the return is late, capped at 12 months. Part of a month counts as a whole one, so being four days late costs the same as being 29 days late.

The part that catches people is that the entity itself usually owes no federal income tax. A pass through passes the tax to the owners. The penalty is for the paperwork, not the money.

Then comes the downstream damage. Your owners cannot file accurate personal returns without K-1s. Miss September 15 by enough and you push every owner past their own October 15 date, where they start running a failure to file penalty on their 1040 at 5 percent of unpaid tax per month, plus interest. One late business return can turn into four late personal ones.

Q3 estimates are due the same day

September 15 is also the third quarter estimated tax payment date for the 2026 year. So on the same Tuesday you close out last year, you should be funding this year. Do the return first if you can, because the K-1 income it produces often changes what your Q3 number should be.

If you are not going to make it, file anyway

This is the part people get backwards. They go quiet, hoping to file a perfect return in November. There is no second extension for a 1065 or an 1120-S. September 15 is the end of the line, and every month of silence is another $255 per owner.

• File with the numbers you have. A return you amend later costs a few hundred dollars in preparer time. Four months of penalty on three owners costs $3,060.
• Ask about first time abatement. If your compliance history for the prior three years is clean, relief is often available on a first offense. It is worth a phone call, not a prayer.
• Small partnerships have a separate path. There is longstanding reasonable cause relief for partnerships with a small number of partners, but it carries conditions and it is not automatic. Do not build a plan around it.

If your bookkeeping is genuinely behind, you can usually get to a defensible return faster than you think. Twelve months of bank and credit card statements, your payroll reports, and your loan statements will carry most of a small entity return. Get it filed, then clean up properly in October when nothing is on fire.

This article is general information for business owners and is not tax advice for your specific situation. Due dates shift for fiscal year filers, short tax years and entities with foreign operations, and penalty amounts are adjusted each year. Confirm your dates with your CPA before relying on them.