Critical Deadlines for Businesses in September 2025

As September 2025 approaches, businesses need to ensure compliance with crucial tax deadlines. Key deadlines this month include filings for S corporations and partnerships with extensions, corporate estimated tax payments, and mandatory payroll tax deposits, all due on September 15.

September 15 - S Corporations
Businesses operating as S corporations must file their 2024 calendar year income tax return (Form 1120-S) and remit any due taxes. This deadline applies if an automatic 6-month extension was previously requested. Furthermore, each shareholder should receive their Schedule K-1 (Form 1120-S) or a corresponding substitute, along with Schedule K-3 (Form 1120-S) if applicable.

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September 15 - Corporations
Corporations adhering to a calendar year are required to deposit their third installment of estimated income tax for 2025. Timely payments ensure compliance and avoid potential penalties.

September 15 - Partnerships
Partnerships should submit their 2024 calendar year return (Form 1065) if a 6-month extension was granted. Partners must receive their Schedule K-1 (Form 1065) or an applicable substitute, and if relevant, Schedule K-3 (Form 1065).

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September 15 - Social Security, Medicare, and Withheld Income Tax
Employers adhering to the monthly deposit rules need to submit their deposits for Social Security, Medicare, and withheld income tax for August 2025 by September 15. This deadline also applies to nonpayroll withholding deposits under the monthly rule.

Weekends & Holidays

If a tax due date falls on a Saturday, Sunday, or legal holiday, the deadline is automatically extended to the next business day that isn’t a legal holiday.

Disaster Area Extensions
In case of disaster area designations, affected deadlines may be extended. For current disaster declarations and relevant extensions, please refer to these resources:

FEMA: https://www.fema.gov/disaster/declarations
IRS: https://www.irs.gov/newsroom/tax-relief-in-disaster-situations

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