The mad dash for April 15 is especially crowded with procrastinating taxpayers this year.
More Americans than usual have waited until the last two weeks to send their tax returns to the Internal Revenue Service. The latest IRS data showed there were about 4.5 million more taxpayers still waiting to file as of March 22 than was the case on that day last year.
Plus, the IRS expects about 2 million more returns this year than last.
Even catching up some ground with late-night number crunching during April's home stretch probably will still leave several million filers staring at the deadline.
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"Everybody's got to file eventually," said Mark Steber, chief tax officer for Jackson Hewitt Tax Service Inc. "This year's like no year we've ever seen."
You can blame the tax season's delayed start for this year's filer-packed finish. Congress rewrote so many tax laws at the last minute -- to avoid the fiscal cliff -- that the IRS was forced to start accepting tax returns two weeks later than normal.
More than 80 million taxpayers have filed returns as of March 22. While that's the vast majority, that's still down 5.2 percent from the same time a year earlier, the IRS said. For those who haven't filed, nearly every tax professional offers the same advice: File. No matter what.
"We have two weeks left. By all means, use (your time to get organized whether you're a do-it-yourselfer or you go into a tax office," said Jackie Perlman, principal tax research analyst for the Tax Institute at H&R Block.
File, because you probably are due a refund. About three out of every four filers
are. This year, the average refund at this stage is $2,827, down 1.2 percent from this time a year ago.
Refunds seem to be running later than usual this year too, said Bret Willoughby of Tax911.com Inc. in Olathe, Kan. The company, which moved here from Wisconsin last summer, specializes in taxes for Americans living overseas.
Willoughby said that a few years ago clients were getting their refunds less than a week after filing electronically.
Refunds do take longer, acknowledged Michael Devine, an IRS spokesman in St. Louis. Those checks now have to pass through a number of tests before heading out the door.
"We're being more careful to catch tax fraud," he said.
The promise at IRS is 21 days for your refund.
File, experts say, even if you don't have all the papers you need from others. For example, some mutual funds and other investment companies have been late sending out 1099 forms that taxpayers need to report gains.
"With a phone call, you may not get the document but you can get the amount," said Steber at Jackson Hewitt.
File, say tax professionals, even if you owe money but can't pay what you still owe. The IRS penalties for failing to file are 10 times bigger than the penalties for paying too little.
Skip the April 15 deadline and Uncle Sam will add 5 percent to your unpaid tax amount every month you are late. Pay too little and the underpayment penalty is 0.5 percent each month until you make up the shortfall.
Besides, the IRS will let you work out a payment plan if you have a financial hardship. Use the online payment agreement at IRS.gov or include IRS Form 9465-FS, "Installment Agreement Request," with your tax return.
File, experts say, even if you can't get your tax return finished in time. Send in Form 4868 to get an automatic six-month extension of time to file.
Note that the extension gives you more time to file. You still have to pay by April 15 any taxes you owe.
So how can you tell how much you owe if you haven't done your taxes yet?
"You make a reasonable effort" to get the amount right, Block's Perlman said. In other words, it's an estimate.
There are a couple of safeguards for filers who don't figure it out by April 15 and file an extension.
The IRS will waive its underpayment penalty if what you have paid in -- from withholdings, estimated payments and any last-minute check to Uncle Sam -- equals at least 90 percent of what your 2012 taxes end up totaling. That's the total 2012 tax bill, not what you still owe on April 15.
If what you still owe come April 15 turns out to be less than $1,000, you also are safe from an underpayment penalty. But if you don't file, that big penalty for failing to file still hits.