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September 2026 Business Tax Due Dates

September brings several critical tax deadlines for small business owners in Gilbert and beyond. From extended returns for S corporations and partnerships to corporate estimated tax payments and employer payroll tax deposits, staying aware of these dates is essential for your business compliance. Review the key due dates below to help keep your business filings and payments on track.

Business tax deadline checklist

September 15: S Corporation Extended Returns

If you requested an automatic 6-month extension for your 2025 calendar year income tax return, your Form 1120-S is due on this date, and you must pay any tax due. Additionally, you need to provide each shareholder with a copy of their Schedule K-1 (Form 1120-S) or a substitute. If applicable, also provide Schedule K-3 (Form 1120-S) or a substitute.

September 15: Corporate Estimated Taxes

Calendar year corporations must deposit their third installment of estimated income tax for the 2026 tax year.

September 15: Partnership Extended Returns

Partnerships that timely requested an automatic 6-month extension must file their 2025 calendar year return (Form 1065) by this date. You must provide each partner with a copy of their Schedule K-1 (Form 1065) or a substitute. If applicable to your partnership, ensure you also provide Schedule K-3 (Form 1065) or a substitute.

September 15: Payroll and Withholding Deposits

If you are an employer subject to the monthly deposit rules, September 15 is the deadline to deposit Social Security, Medicare, and withheld income tax for August 2026. This is also the due date for the August 2026 nonpayroll withholding deposit if the monthly deposit rule applies.

September 30: Fiduciary Returns for Estates and Trusts

If you were granted a 5½-month extension, your 2025 calendar year return (Form 1041) is due on September 30. If applicable, you must provide each beneficiary with a copy of their Schedule K-1 (Form 1041) or a substitute Schedule K-1.

Rules for Weekends and Legal Holidays

Whenever a standard due date falls on a Saturday, Sunday, or a legal holiday, the deadline is automatically extended to the next business day that is not a legal holiday.

Extensions for Designated Disaster Areas

When a specific geographic location is federally designated as a disaster area, tax deadlines are generally extended. To check if an area has been designated as a disaster area and to review applicable filing extension dates, visit the following resources:

Stay on Top of Your September Deadlines

Managing these due dates is a vital part of running a compliant small business. If you need assistance preparing your extended returns, calculating estimated payments, or handling payroll compliance, the team at Martinez & Shanken PLLC is here to help. Schedule a consultation today to keep your business finances running smoothly.

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Gilbert, Arizona 85233
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