Four dates a year, and what happens after one slips.

The four dates and the months behind them

The IRS divides the year into four payment periods. Income from January 1 through March 31 is due April 15. Income from April 1 through May 31 is due June 15. Income from June 1 through August 31 is due September 15. Income from September 1 through December 31 is due January 15 of the following year.

Look at the lengths: three months, then two, then three, then four. The June payment follows a short period, so it often feels like it comes right after the April one. The January payment covers the longest stretch and lands after the holidays.

When a due date falls on a Saturday, Sunday, or legal holiday, a payment made on the next business day counts as on time. If you mail a payment, the IRS treats the U.S. postmark date as the date paid.

How the payments get made

The IRS accepts estimated payments online, through your IRS online account, by phone, through the IRS2Go app, or by mail with a Form 1040-ES voucher. Form 1040-ES also includes a worksheet for figuring the amount, starting from your expected income, deductions, and credits for the year.

If a payment is missed or comes up short

Paying too little, or paying late, can bring the underpayment of estimated tax penalty. The IRS figures it from three things: how much was underpaid, how long it stayed underpaid, and the quarterly interest rate the IRS publishes for underpayments. Because time is part of the formula, a shortfall paid sooner costs less than the same shortfall left until April.

The penalty is figured period by period, so a strong fourth quarter does not automatically erase a thin second one. It can apply even if you end up due a refund when you file.

Form 2210 is how the penalty is worked out, or shown not to apply. If your income arrived unevenly, the form also offers a way to match what was due to when the income actually came in, which can shrink or remove the penalty for people whose big months came late in the year.

When the penalty generally does not apply

The IRS lists two main ways to avoid it. One is if your return shows you owe less than $1,000 after withholding and credits. The other is if you paid, through withholding and estimated payments, at least 90% of this year's tax or 100% of last year's, whichever is smaller. If last year's adjusted gross income was over $150,000, the prior-year figure becomes 110%.

Paying at least last year's total tax, split across the four dates, is a common approach for people whose income swings, because it is a number already known in January. It covers the penalty question, though not necessarily the full bill if income rises.

The IRS may also reduce or remove the penalty after a casualty, a disaster, or another unusual circumstance, and in some cases after retiring at 62 or older or becoming disabled.

Keeping the dates from sneaking up

The practical fix is having the money ready before the date rather than finding it on the date. Setting aside a share of each payment as it arrives means the quarterly amount is already sitting there. The set-aside calculator shows what that share and the per-quarter figure look like for the income you expect.