The 90-day window that decides your tax bill

Every year we take calls in February from business owners who want to reduce the tax on a year that ended six weeks earlier. Almost always, the honest answer is that the decisions were made in October and the window has closed. Here is what that window contains.

Why the fourth quarter is different

Tax follows the calendar with very little sympathy. Most levers depend on something happening: an asset placed in service, a plan adopted, a payment made, a distribution taken, and once 31 December passes, that event either occurred or it did not.

The fourth quarter is also the first point in the year when you have enough information to plan well. By late September you have three quarters of actual results and a defensible forecast for the fourth. Planning in June relies on guesses; planning in January is archaeology. October to December is where reliable data and available options overlap, and it is a narrow overlap.

What is still open in Q4

Fixed asset timing. Depreciation requires the asset to be placed in service, ready and available for its intended use, by 31 December. Delivery on 28 December with installation in January does not qualify. Ordering equipment in November with an eight-week lead time is a decision about next year's return, not this one.

Retirement plan design. This is frequently the largest single lever available to a profitable owner-managed business. Some plan types must be adopted by year-end even where funding can follow later, and a cash balance plan alongside a 401(k) can support deductible contributions several times a standard limit for an owner in their fifties. Design takes weeks, so October is the moment to start, not December.

Income and expense timing. For cash-basis taxpayers, deferring December invoicing or prepaying deductible expenses shifts income across the year end. This is genuinely useful when rates or income levels differ between years, and merely cosmetic when they do not. It is worth being clear which situation you are in.

Capital gains and losses. Harvesting losses to offset realized gains has to happen by 31 December, with the wash sale rules watched carefully.

Charitable giving. Bunching two years of giving into one via a donor-advised fund, and contributing appreciated securities rather than cash to avoid the embedded gain entirely.

Basis and distributions. Confirming shareholder basis before a year-end distribution. A distribution exceeding basis is taxable gain, and finding that out in March is an unpleasant surprise that was entirely avoidable in November.

Reasonable compensation true-up. An S-corp owner whose wages are low relative to a strong year can still run a final payroll adjustment in December.

What has already closed

Equally important, and less often discussed:

• Entity elections for the current year. Form 2553 for S status had a 15 March deadline for the year in progress. In Q4 you are electing for next year.
• Section 83(b) elections. Thirty days from grant, no extensions, no reasonable cause relief. If the window passed, it passed.
• 1031 exchange identification. Forty-five days from the closing of the relinquished property.
• Anything requiring a transaction to have already happened. You cannot retroactively restructure a sale that closed in July.

Knowing which list an item belongs on is most of the value of a Q4 review. Time spent on the second list is time wasted.

The estimated payment question

Q4 is also when the estimated payment position should be settled. The safe harbor eliminates the underpayment penalty: pay 100% of last year's total tax through withholding and estimates, 110% if prior-year AGI exceeded the threshold, and no penalty applies regardless of how much larger this year turns out to be.

Two practical notes. Estimated payments are credited by quarter, so a large January payment does not cure an underpayment from April; the penalty is computed period by period. But withholding is treated as paid rateably across the year regardless of when it occurred, which means increasing withholding on a December payroll can retroactively fix an underpayment that quarterly estimates cannot.

A practical Q4 sequence

1. Late September: close the third quarter properly and build a full-year projection. Everything else depends on a number you trust.
2. Early October: review the projection against the prior year. Identify which levers are genuinely available and quantify each one.
3. Mid October: start anything with a lead time, such as retirement plan design, equipment with long delivery, or a cost segregation study.
4. November: confirm basis, settle the compensation position, and decide the giving strategy.
5. December: execute. Place assets in service, adopt plans, run the final payroll, make the distributions and the gifts.

None of this is exotic. It is simply a matter of doing the arithmetic while the answers can still change something, which is the entire difference between tax planning and tax reporting.