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Key Tax Deadlines for Businesses in September 2025

As we step into September 2025, business owners need to be well-prepared for a series of critical tax deadlines. From S corporation and partnership returns to corporate estimated payments and payroll tax deposits, understanding these due dates can help your business stay compliant and financially healthy.

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September 15 - S Corporations
If you elected an automatic six-month extension earlier this year, it's time to file your 2024 calendar year income tax return (Form 1120-S) and settle any tax liabilities. Be sure to provide each shareholder with their Schedule K-1 (Form 1120-S) or a substitute, and if relevant, a Schedule K-3 (Form 1120-S) or its substitute.

September 15 - Corporations
Calendar year corporations should deposit the third installment of their estimated income tax for 2025. Staying on top of these payments is crucial to avoid penalties and interest.

September 15 - Partnerships
Similar to S corporations, partnerships with an automatic extension need to file their 2024 calendar year return (Form 1065) by this date. Partners should receive their Schedule K-1 (Form 1065) or substitute, along with Schedule K-3 (Form 1065) or its equivalent if applicable.

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September 15 - Social Security, Medicare, and Withheld Income Tax
Employers must adhere to the monthly deposit rules and remit their August 2025 deposits for Social Security, Medicare, and withheld income taxes by this date. This deadline also applies to nonpayroll withholding deposits following the monthly deposit rule.

Weekends & Holidays
When due dates coincide with a Saturday, Sunday, or legal holiday, they automatically extend to the following business day.

Disaster Area Extensions
Regions declared as disaster areas enjoy extended filing deadlines. Check for designated areas and deferment dates through these resources:
FEMA Disaster Declarations
IRS Tax Relief in Disaster Situations

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