September Tax Deadlines Ahead

Two big deadlines land on the same day this month — here’s what’s coming so you’re not caught off guard.

Quick Answer

September 15, 2026 is the date to circle. If your S-corp or partnership filed a six-month extension back in March, that extension runs out on September 15 — extended Form 1120-S and Form 1065 returns are due. The same day is also the deadline for your Q3 2026 estimated tax payment. Two obligations, one date, and if either slips, the penalties start running immediately.

What’s Due on September 15

• Extended S-corp returns (Form 1120-S). If your S-corp filed Form 7004 by the March 16 deadline, the six-month extension expires September 15. Schedule K-1s need to go out to shareholders the same day.
• Extended partnership returns (Form 1065). Same extension window, same due date. K-1s for partners are due alongside the return.
• Q3 2026 estimated tax payments. If you pay quarterly — as a self-employed business owner, real estate investor, or shareholder receiving pass-through income — your third installment for the year is due the same day.

Because S-corp and partnership income flows through to the owners’ personal returns, this date tends to set off a chain reaction: the K-1 you receive on the 15th becomes an input for your own 1040, which is still on extension until October 15.

Why This Date Catches People Off Guard

Two things make mid-September different from a typical due date:

• An extension moves the paperwork deadline, not the payment deadline. Any tax owed on the extended S-corp or partnership return should already have been paid back in March — September 15 is about the return itself.
• The Q3 estimate is a separate, unrelated obligation that happens to land on the same calendar day. It’s easy to handle one and forget the other.

If you’re a shareholder or partner who also pays personal estimated taxes on that pass-through income, you’re effectively juggling two deadlines that share a due date but nothing else.

How to Stay Ahead of It

• Confirm your entity’s extension status. If you’re not sure whether Form 7004 was filed in March, check now — not on September 14.
• Get final numbers to your bookkeeper early. Clean, reconciled books make the difference between a same-day filing and a scramble. The closer your books are to done before September, the smoother this goes.
• Calculate Q3 based on where the year actually stands. If your extended return is nearly finished, you have real year-to-date numbers to base the Q3 payment on, rather than a guess from last spring.
• Don’t let a missing document stall everything. If you’re waiting on a K-1, a 1099, or reconciled bank statements, file the most accurate return you can on time rather than missing the deadline altogether.

Frequently Asked Questions

What happens if I miss the September 15 deadline?

For extended S-corp and partnership returns, the IRS charges a per-shareholder or per-partner penalty for each month the return is late, up to 12 months. For the Q3 estimated payment, interest accrues daily on the underpayment until it’s paid.

Do I still need to pay if I already filed my extension?

The extension only pushed back the deadline to file — not to pay. Any tax owed on the S-corp or partnership return should have been paid with the extension in March. September 15 is about submitting the completed return itself.

I’m an S-corp shareholder — do I owe two things on the same day?

Possibly. Your S-corp’s extended return is due September 15. Separately, if you personally pay quarterly estimated taxes on your share of that income, your Q3 installment is also due September 15. They’re related but calculated and paid separately.

What if my K-1 isn’t ready by the 15th?

Talk to your bookkeeper or tax preparer as early as possible. Filing an accurate return with the information available is almost always better than missing the deadline while waiting on a document.

About the Author

Jenny Rodriguez is the founder of Anchor Bookkeeping & Tax Solutions and a QuickBooks Platinum ProAdvisor with more than 10 years of experience helping construction contractors, real estate investors, and small business owners across Charlotte, NC keep their books audit-ready and their tax deadlines stress-free. Jenny is bilingual in English and Spanish.

Get Ahead of Your September Deadlines →

Not sure if your books are ready for September 15? Anchor Bookkeeping & Tax Solutions helps Charlotte-area businesses stay ahead of every deadline, all year long. Reach out today to get your books in order.

Disclaimer: This article is provided for general informational purposes only and does not constitute tax, legal, or accounting advice. Deadlines and rules referenced apply to calendar-year filers under current IRS guidance and are subject to change. Please consult a qualified tax professional regarding your specific situation.