September brings several important tax deadlines that individuals, dental practices, and other business owners should keep on their radar. Staying ahead of these dates helps avoid penalties, keeps cash flow predictable, and ensures compliance with IRS requirements. Below is a breakdown of the most relevant deadlines this month.
September 10 – Reporting Employee Tips
If you or your staff members receive tips, remember that tips totaling more than $20 in August must be reported to the employer by September 10 using IRS Form 4070. Employers then calculate and withhold the appropriate income tax and FICA (Social Security and Medicare) contributions from wages.
If wages are not high enough to cover the required withholdings, the employer will note the uncollected amounts on the employee’s Form W-2 (Box 8). These amounts will still need to be paid when the employee files their annual tax return.
September 15 – Third Quarter Estimated Tax Payments
The third installment of 2025 estimated taxes is due for individuals and calendar-year businesses. The U.S. operates on a pay-as-you-go tax system, so taxpayers who don’t have enough income tax withheld through wages or retirement distributions must make estimated payments. This often applies to dentists and practice owners who receive income through self-employment or pass-through entities.
Failing to make sufficient prepayments can trigger underpayment penalties. Fortunately, the IRS provides two “safe harbor” options to avoid penalties:
- Current-Year Method – Pay at least 90% of the tax you’ll owe for 2025.
- Prior-Year Method – Pay 100% of your 2024 tax liability (110% if your adjusted gross income exceeds $150,000, or $75,000 if married filing separately).
For practice owners, this is especially important if your income fluctuates due to practice growth, a property sale, or other large transactions. If you’re unsure whether your payments are sufficient, we recommend reviewing your numbers with a CPA before the deadline.
September 15 – S Corporations and Partnerships
Businesses structured as S corporations or partnerships that requested a six-month extension have until September 15 to file their 2024 tax returns:
- S Corporations (Form 1120-S): File the return, pay any tax due, and distribute Schedule K-1 (and if applicable, Schedule K-3) to shareholders.
- Partnerships (Form 1065): File the partnership return and provide partners with Schedule K-1 (and if applicable, Schedule K-3).
September 15 – C Corporations Estimated Tax Payment
Calendar-year C corporations must also submit their third installment of estimated income tax for 2025 by this date.
September 15 – Payroll Tax Deposits
Employers who follow the monthly deposit schedule must deposit Social Security, Medicare, and withhold federal income tax for August 2025 by September 15. This also applies to monthly deposits of non-payroll withholdings.
Weekend, Holiday, and Disaster Area Adjustments
- Weekend & Holiday Rule: If a tax deadline falls on a Saturday, Sunday, or legal holiday, the due date automatically moves to the next business day.
- Disaster Relief Extensions: In federally declared disaster areas, taxpayers may receive additional time to file and pay. For the most current updates, visit FEMA or the IRS disaster relief page.
Plan Ahead
September is one of the busiest months for tax deadlines, especially for dental practices balancing payroll, estimated taxes, and entity filings. Proper planning helps avoid surprises and penalties.
Need help navigating these deadlines or ensuring your estimated payments are accurate? Contact DentalCPA now to schedule a tax planning consultation.