Corporate tax deadlines don’t always land where practitioners expect them. Some are obvious (the filing deadline). Others are buried in code sections that only matter once a year, at exactly the wrong time. Missing them rarely triggers a dramatic penalty. It triggers a missed deduction, a forfeited election, or an underpayment that compounds quietly.
Five deadlines consistently catch practitioners who handle corporate returns.
1. The Bonus Accrual Window: March 15
An accrual-basis C corporation can deduct a bonus in the current tax year only if the amount is fixed by December 31 and paid within 2.5 months after the close of the tax year. For calendar-year corporations, that’s March 15. A bonus approved by the board in December but paid on March 20 becomes a deduction in the following year. Five days is the difference between a current-year deduction and a missed one.
2. The September 15 Estimated Tax Installment
Corporations expecting to owe $500 or more in federal income tax make quarterly estimated payments. The third installment falls on September 15, and it’s the last clean adjustment point before year-end. For large corporations (taxable income of $1 million or more in any of the prior three years), the prior-year safe harbor only covers the first installment. After that, payments follow current-year projected tax. A Q3 shortfall compounds daily at the underpayment rate, which sat at 7% for Q1 2026 (IRS Form 2220).
3. The “Placed in Service” Cutoff: December 31
Depreciation deductions (including 100% bonus depreciation and Section 179 expensing) don’t begin when the asset is purchased or shipped. They begin when the asset is placed in service, meaning it’s available and ready for use in the business. Equipment ordered in November that arrives in January generates a 2027 deduction, not 2026. For assets with meaningful lead times, the effective ordering deadline is often September or October, not December.
4. Retirement Plan Contribution Deadlines
C corporations can deduct employer retirement plan contributions of up to 25% of covered compensation, but only if funded by the tax return filing deadline (including extensions). For calendar-year C corps, that’s April 15 or October 15 with an extension. The 2026 annual additions limit under Section 415(c) is $72,000 per participant, plus $8,000 in catch-up contributions for those 50 and older. Missing the funding deadline doesn’t just lose the deduction. It can create compliance issues with the plan itself (IRS Publication 560).
5. Charitable Contribution Timing
Corporate charitable deductions are capped at 10% of taxable income, computed before the deduction and certain other adjustments. Contributions exceeding the cap carry forward five years. But the timing of recognition matters: accrual-basis corporations can deduct a contribution in the current year if the board authorizes it by December 31 and payment is made within 3.5 months after year-end (by April 15 for calendar-year corporations).
The 10% ceiling also interacts with other planning decisions. Large bonus payments or accelerated depreciation deductions can reduce taxable income enough to lower the charitable contribution cap, creating a cascading effect that isn’t visible until the return is prepared.
The Common Thread
All five of these deadlines share a pattern: they’re mechanical, not discretionary. Missing them doesn’t trigger a judgment call. It triggers a fixed consequence. The deduction shifts to next year, the penalty accrues, or the election is forfeited. For practitioners who handle corporate returns, the Q3 window is when all five get reviewed.