Nine tax deadlines mistakes around extensions, estimates and Form 941

Deadline mistakes are rarely about forgetting a famous date. They are about not knowing an obligation existed, misunderstanding what an extension does, or discovering on the day that the money is not there.

What to take away

Why deductions and mileage depend on the date

Deductions and mileage records only help when they reach a filed return. A mileage log and a deduction schedule are evidence, and the calendar decides when that evidence must be used.

If you extend to file, you still need to estimate and pay, and your deduction and mileage records should be current before you do. A missed deadline can leave good records sitting unused.

Where three of the nine land on the calendar

Mistakes 1, 3 and 4 attach to fixed federal dates. The table lists those dates with the mistake each one invites; the other six do not sit on a fixed date. The tax deadlines overview explains how the calendar is structured.

A quarterly check against the table

Before each date in the table, run four questions. They catch the mistakes that a reminder alone will not.

The first and second questions need the most care. Estimates are the obligation people postpone when the year looks uncertain, and new dates arrive without notice. Write each answer down with the date you checked, so the next quarter starts from a record rather than from memory.

Mistakes tied to a date

1. Treating an extension as more time to pay

The most expensive misunderstanding in the subject. Form 4868 buys more time to file and does nothing to the payment date. Late filing and late payment are two separate penalties, charged at different rates. Pay an estimate when you extend, through Form 1040-ES or withholding.

The late filing penalty is 5% of the tax due for each month or partial month the return is late, up to a maximum of 25%. When both penalties apply, the late filing penalty is reduced by the late payment penalty of 0.5% for each month.

For a 2025 calendar year return, Form 4868 is filed by the due date of the Form 1040, which is April 15, 2026, for most people.

The extension generally can't run more than 6 months, to October 15, 2026, for most calendar year taxpayers. Tax not paid by April 15, 2026, starts the late payment penalty for each month or part of a month it stays unpaid.

3. Skipping estimated payments because the year looks uncertain

Uncertainty is the reason to estimate, not a reason to wait. Form 1040-ES is the form the payments are made on. For 2026 the four due dates are April 15, 2026, June 15, 2026, September 15, 2026, and January 15, 2027.

The January payment can be skipped if the 2026 return is filed by February 1, 2027, with the entire balance paid.

The estimated tax safe harbor in Publication 505 is the number to plan around. For 2026 you generally pay the smaller of 90% of your total expected tax for 2026 or 100% of the tax shown on your 2025 return.

If your 2025 AGI was more than $150,000 ($75,000 if married filing separately), 110% replaces 100%. Read the publication rather than guessing at the numbers.

4. Missing the deposit calendar entirely

Employment deposits run on a schedule that is determined by rules rather than chosen, and it does not pause because a return is extended. Form 941 is the quarterly employment tax return those deposits belong to.

The IRS publishes the schedule on its employment tax due dates pages. Form 941 is due January 31, April 30, July 31 and October 31. Employers who deposit the quarter's tax in full and on time have an additional 10 days to file. Forms W-2 go to each employee by January 31.

Mistakes with no date on the calendar

2. Not knowing an obligation exists

A new state, a new employee, a new registration. Each brings its own dates and none of them announce themselves. This is the most common cause of lateness in a growing business.

5. Depending on someone else's document without a plan

If your return needs a statement from another business, their timetable is your timetable. A Form 1099 or a Schedule K-1 arrives when it arrives; assuming yours will come on time is not a plan, and asking for a date and preparing to extend is.

6. Not reading agency mail

Notices carry dates that start clocks. An IRS notice or a letter from your state tax agency is a dated document, not a formality, and an unopened envelope in a drawer has cost more businesses more money than any calculation error.

7. Treating state and federal as one calendar

They are separate systems with separate dates, and one being handled says nothing about the other. Most states run a tax agency for income and sales tax and a separate workforce agency for unemployment insurance, each with its own filing calendar again.

8. Filing on a guess to hit a date

A return with an invented number on it is not a filing achievement. Extend, estimate the payment properly, and file something you can support.

Mistake 9, and the pattern behind the nine

9. Fixing lateness with resolve rather than with process

Promising to be earlier next year changes nothing. The causes are structural: bookkeeping behind, no named owner of the calendar, or a dependency nobody manages.

Two of the nine are genuinely about understanding a rule. Everything else is about knowing what exists and having somebody own it.

Common questions

Which of the nine is worst?

Number 1 for cost per occurrence, and number 4 for severity, because employment deposits involve money that was never the business's.

We missed something last year and nothing happened.

Nothing has happened yet. Correspondence can arrive well after the fact, and the fix is cheaper before it does.

Can I set up my own reminders and be done?

Reminders fail on the obligations you do not know about, which is mistake 2. Rebuild the list annually from the authorities' own pages, then set reminders from that.

Our accountant handles the calendar.

Confirm which parts. State registrations and employment agency filings are the usual gaps, and both sides often assume the other has them.