Heidi Hutner, a writer and professor, likes to file her taxes as early as possible. This year, it didn't go quite to plan. 

"I started on time, and then found it hard to get online to track down a few things," Hutner said. Between working on a book, shooting a film and falling sick twice in March, she put off gathering some key documents later and later. She finally got the materials to her accountant on Thursday.

"She was pissed," Hutner said.

While rare for Hutner, this experience isn't so unusual in America. About one in five tax filers waits until the last two weeks before the filing deadline to file their taxes. 

"We process about a third of all volume in April," said a spokesperson for the Internal Revenue Service. That includes the people who file extensions in April and the return itself later in the year. 

When is the last day to file taxes in 2018?

The tax filing deadline this year is Tuesday, April 17. That's because the 15th is a Sunday and the following Monday is a federal holiday.

Last year, the deadline fell on April 18. That year, 17 million returns—11 percent of the total—were filed that very week. Another 10 percent came in the week just before. The back-loading could be even more extreme this year, as the filing season started nearly a week later and people receiving some tax credits couldn't file until mid-February. 

The IRS processes about 152 million individual returns every year. As of April 6, just over 103 million had been filed. That means nearly 50 million taxpayers have either spent the last week scrambling to locate key documents and file their returns, or have scrambled to locate key documents and instead filed for an extension. 

Nearly 1 in 10 tax filers takes advantage of an even further delay that IRS automatically grants: the 6-month extension. About 14 million Americans will get an extension this year, the IRS said in a press release.

An extension, however, only serves to delay any refund you have coming; any tax owed must be paid by Tuesday.

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