An illustration of an IRS 1040 tax form

Monday, April 15, is the deadline for most Americans to file their 2018 federal and state income tax returns. Massachusetts and Maine residents get two more days to file, on Wednesday, April 17, thanks to state holidays.

Don't panic if you're still not ready. The Internal Revenue Service offers lots of options for procrastinators, from flexible payment options to extensions. 

And you won't be alone in seeking this help. Last tax season, taxpayers submitted 14.7 million tax extension forms, the IRS says.

How to File for a Tax Extension

If you haven't finished preparing your taxes, you should file for an extension by midnight on the tax deadline day. The IRS’s Free File online system allows you to prepare and electronically file IRS Form 4868, the extension application, free of charge. Tax-preparation software, like TaxAct and H&R Block, lets you do the same.

More on Taxes

An extension will give you until October 15 to file your actual tax return, but it doesn't give you a break on paying your taxes. You'll still have to estimate what you owe for 2018—and pay the IRS—along with submitting your Form 4868 by midnight on tax day. You'll need to do the same for anything you owe your state.

If you don't pay on time, the IRS will add penalties and interest to the original amount due. Currently, the annual interest rate is 6 percent, compounded daily.

If you're expecting a refund from the IRS for the income taxes you already paid in 2018, there's no penalty associated with getting a tax extension, but the government will hold your money until you file. 

And if you fail to file your tax return within three years of the due date, the government gets to keep any refund. According to the IRS, Americans will leave about $1.4 billion on the table by not filing for what they're owed.

Who Gets an Automatic Extension

A few folks get an automatic extension to file their taxes, without having to apply. They include members of the military and eligible support personnel serving in a combat zone, and residents in areas that have been deemed official national disaster areas. (This season, for instance, victims of catastrophic storms, winds, and flooding that took place in March in Nebraska will have until July 31 to file federal income tax returns and other tax documents normally due between March 9 and July 31.)

Permanent residents living abroad have a two-month extension to file. However, they still must pay their estimated tax bill by Monday, April 15, to avoid being charged interest.

What's the Drop-Dead Time to File?

If you're filing online, your return must be filed before midnight on Monday—or Wednesday, if you're in Maine or Massachusetts. "If a return is time-stamped by 11:59 p.m. local time when filing via our offices or online, taxpayers are fine," notes Kevin Martin, tax research analyst at H&R Block.

You'll get an email from the tax software company indicating that your return has been filed, which serves as the time stamp. The fact that the IRS actually may receive your return after midnight is immaterial, Martin says.

That said, it's better to file earlier in the day rather than later. "You don’t know what might happen that is beyond your control," Martin adds. 

Occasionally the IRS will reject your return, usually due to a typo or math error. The agency will notify you by email, though it may take a day or two. 

"In that case, you generally have a 5-day grace period—called the 'perfection period'—to electronically resubmit the return," Martin explains. "For example, if the IRS rejects a return transmitted on April 15, the taxpayer has until April 20 to resubmit the return."

Maine and Massachusetts residents whose returns are rejected on April 17 have until April 22 to resubmit their returns. 

If you choose not to resubmit the return electronically and instead paper-file the return, you have 10 days after the e-filed return is rejected to file on paper. 

Paper returns are considered filed at the time the envelope is postmarked. Ask for a receipt from the postal clerk. Use the U.S. Postal Service's Postal Locator to find post offices open late on tax day.

What About Contributing to an IRA?

Contributions to traditional and Roth IRAs, as well as SEP-IRAs, other retirement accounts, and health-savings accounts, can be applied to your 2018 taxes if they're made by midnight on April 15—or April 17 for Maine and Massachusetts residents. You must indicate that the contribution is for tax-year 2018.

Maura Cassidy, vice president of retirement at Fidelity Investments, notes that customers who transfer money or send it through a mobile check deposit to the investment house will get a confirmation by mail or email. Mobile check deposits will be accepted up to 11:59 p.m. on the tax deadline day.

Be sure to note that it's a "prior year contribution for 2018." Regardless of whether you immediately invest the money or leave it in cash to be invested later, you'll get credited for tax-year 2018, making you potentially eligible for a tax deduction.

(If your transaction is completed after 4 p.m. but before midnight on Monday, it will be recorded as a contribution to your 2018 IRA, though any investment of that money will take place on Tuesday, April 16.) 

If you mail an IRA contribution check to an investment company, the same policy applies as to mailed tax returns: The date of the postmark is what counts, regardless of the date that the investment company receives the money from your account. "That said, it's best to date the check no later than April 15," Cassidy says.

Remember to also get and retain proof of your postmark if you mail your contribution close to April 15, Cassidy adds.