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Monday, April 18, is the deadline for filing your 2016 tax return. It's another deadline, too -- and one that could cost you. (Why not April 15? Well, because Washington, D.C., observes Emancipation Day on the 15th, and the 16th and 17th fall on the weekend. Thus, the main tax deadline gets pushed forward for most folks. For those in Maine and Massachusetts, it's April 19, because  of Patriots' Day.)

But let's get back to that you-might-lose-money deadline. It has nothing to do with the 2015 tax return that's due on the 18th and instead is tied to your 2012 taxes. And it may apply to you chiefly if you did not file a tax return. Why would anyone not have filed a return? Well, we don't all have to file returns. If you earn less than a certain sum, you don't have to file a return. That sum, for the 2015 tax year, is $10,300  if your filing status is single and you ended the year younger than 65 and $11,850 if you were 65 or older at the end of 2015. It's higher for heads of households and married folks, but it's still on the low side -- peaking at $23,000 for couples where both parties are 65 or older.

You may also have not bothered to file a tax return if you earned more than the minimums above but knew that you had had enough taxes withheld so that you didn't owe anything. If you owe no taxes, you don't have to file a return -- though, again, it's often worth doing so.

Regardless of why you didn't do so in the past, filing a late tax return may still be the smart thing to do. That's because you may be owed a refund! After all, most wage earners have taxes withheld from paychecks and if you end up having paid more than you needed to over the year, you'll want to get back as much as you're entitled to. Not filing a return, though, means no refund.

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Deadline approaching
The April 18 deadline is for those who did not file a 2012 tax return. If you didn't file one for 2013 or 2014, you have more time to do so -- and doing so can be worthwhile, as you may have refunds due you for those years, too. But let's focus on 2012, because you need to hurry. If you miss the deadline, Uncle Sam will keep what had been rightfully yours.

The IRS has released information on how much is due to Americans. About a million taxpayers fall into this group of non-filers due a refund, with the unclaimed refunds totaling about $950 million. The median refund owed to non-filers is $718, meaning that plenty of folks are due much more than that (and others due less).

Want more incentive to file that late return? Well, if you earned little income in 2012, you may be eligible to claim the Earned Income Tax Credit (EITC) for that year -- and in 2012, the EITC was worth up to $5,891 .

Here's a last bit of good news: There's no IRS penalty for filing a tax return late if you didn't owe any taxes. If you're simply due a refund, you can file a late return without any punishment. (Note: If you owe back taxes, or are in arrears with regard to child support or federal student loans, your refund will likely be withheld.)

A last detail to know is that if you are going to file for your 2012 refund, the IRS expects you to have filed returns for 2013 and 2014, as well. Fail to do so and your refund check may  be held.

Any sum near $718 is significant for most of us. It might represent several months' worth of food costs or half a month of rent. Don't leave it on the table for Uncle Sam to pocket.