The April 18 deadline for filing federal income tax returns is less than three weeks away. Taxpayers who think that they will not complete their returns on time may want to consider filing extensions.

A quirk of the calendar is giving taxpayers more time to file their returns this year. (The traditional filing day, April 15, falls on a Saturday, but the next Monday is a holiday in Washington, bumping tax day to Tuesday.)

Even with the extra three days, however, millions of taxpayers will not file on time — whether because of simple procrastination, or because they are missing crucial forms. Last year, 13 million taxpayers requested extensions, the I.R.S. said.

Filing for an extension is easy. Melissa Labant, director of tax advocacy with the American Institute of Certified Public Accountants, said almost anyone could get an automatic, six-month extension to file by filling out and submitting Internal Revenue Service Form 4868. Requests, unlike those in years past, require no explanation, she said.

While getting an extension is simple, tax experts emphasized a cardinal rule: An extension buys more time to file your return, but you still must pay any taxes you owe by the initial deadline.

“It’s an extension of time to file, not to pay,” said Bernadette Antonelli, an enrolled agent with Arlington Tax in Kearny, N.J. Enrolled agents are federally licensed tax preparers.

If you owe taxes and do not pay by the filing deadline, you will be subject to penalties and interest. So if filers think they will owe money, they should estimate the amount the best they can, Ms. Antonelli said, and make the payment when they file for the extension.

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