If your business operates as an S-corporation, a partnership, or a multi-member LLC taxed as a partnership, September 15 deserves a spot on your calendar. It's the date your extended 2025 tax return is due if you filed Form 7004 back in the spring — and it's also the deadline for your third-quarter estimated tax payment. Two obligations, same day. It's easy to let the filing deadline absorb all the attention and forget that it's also your last real checkpoint for retirement plan decisions tied to last year.

Two Deadlines Collide on September 15

For calendar-year S-corporations (Form 1120-S) and partnerships (Form 1065) that requested a six-month extension, the return is due September 15, 2026. If you're a partner or S-corp shareholder who also pays estimated taxes personally, your third-quarter 2026 estimate is due the same day. Neither deadline extends the other, and the extension to file was never an extension to pay — any tax owed on the original return should already have been paid back in March.

Why This Date Also Matters for Retirement Plan Funding

Here's the piece that catches business owners off guard: for many retirement plans, the deadline to establish and fund the plan for a prior tax year follows your business's tax filing deadline, including extensions. If your S-corp or partnership return is due September 15, that's typically also your deadline to fund a SEP-IRA for last year based on that entity's compensation.

A couple of nuances worth knowing:

  • SEP-IRAs are straightforward here — both establishing the plan and making the contribution can generally happen up until the extended filing deadline.
  • Solo 401(k) plans are more layered, and the rules differ depending on how you're paid. If you're a W-2 employee of your own S-corp, your deferral election generally needs to be in place before the wages are paid — in practice, by December 31 of that plan year. The employer profit-sharing side can still follow the extended filing deadline. Don't assume the September 15 window applies to every dollar in a Solo 401(k) — confirm the specifics with your tax preparer.

If You're a Sole Proprietor, Your Clock Runs Longer — Mostly

Sole proprietors and single-member LLCs filing on Schedule C don't file a separate business return — your business activity flows onto your personal Form 1040. If you extended your personal return, your deadline is October 15, 2026, not September 15. That extra month applies to establishing and funding a SEP-IRA, and it applies to depositing Solo 401(k) contributions.

One important exception: if you're deciding how much to defer as the "employee" side of a Solo 401(k), that election generally needs to be locked in by your unextended filing deadline — April 15, 2026 for last year's decision — even though you have until October to actually deposit the money. Filing an extension buys you more time to fund, not more time to decide. This is a detail worth confirming with your tax preparer before you assume October 15 covers everything.

The 2025 Numbers Worth Knowing

Since the contribution you're funding by these deadlines is for the 2025 tax year, it's the 2025 limits that apply — not this year's numbers:

  • A SEP-IRA contribution for 2025 is capped at the lesser of 25% of compensation or $70,000, based on compensation up to $350,000.
  • A Solo 401(k) employee deferral for 2025 is capped at $23,500, with an additional $7,500 catch-up available for those age 50–59 or 64 and older, and a larger $11,250 catch-up for those age 60–63.
  • Combined employee and employer contributions to a Solo 401(k) for 2025 top out at $70,000 (before catch-up amounts).

(For reference, all of these limits step up for 2026 — the SEP-IRA and combined Solo 401(k) caps rise to $72,000, the compensation cap to $360,000, and the employee deferral to $24,500 with an $8,000 standard catch-up — but those higher figures apply to money you're setting aside for the current year, not the 2025 contribution tied to this deadline.)

These are IRS limits, not recommendations — how much makes sense for your business depends on your cash flow, your income for the year, and your broader financial picture. A tax or financial professional can help you work through the actual math for your situation.

Treat It as a Checkup, Not Just a Filing Deadline

The value in this date isn't just meeting it — it's using it as a forcing function. If you're pulling together numbers for your extended return anyway, that's a natural moment to ask a few broader questions: Is your entity structure still the right one as the business has grown? Are you funding a retirement plan at a level that reflects this year's actual profitability, or just repeating what you did last year? Is there still time to make a meaningful contribution before the window closes?

For cross-border business owners with income or entities on both sides of the US–Mexico border, this is also worth reviewing alongside your Mexican tax obligations, since deadlines and retirement vehicles don't line up between the two countries. If you haven't had someone look at your business's tax and retirement plan timeline together this year, September is a good time to have that conversation — before the deadline decides for you.

This article is for general educational purposes and does not constitute tax, legal, or individualized financial advice. Retirement plan deadlines and rules vary by plan design and entity type — consult a qualified tax or legal professional about your specific situation.