Oct. 17 isn't the first date that comes to mind as being an important deadline. But for many people, Monday is their last chance to do two things: Get their 2015 taxes filed, and undo a Roth IRA conversion from last year. Below, we'll look at why even those taxpayers who got their taxes done back in April should take a look and see if there's an opportunity to get some smart tax planning done in the next three days.

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Time's up on 2015 extensions (except for Hurricane Matthew victims)

Back in April, millions of taxpayers asked for extensions to get more time to file their tax returns. The IRS has made filing for a tax extension extremely easy, with automatic extensions granted to anyone who made the appropriate request on IRS Form 4868 before this year's deadline of April 18. The extension gives you six extra months to file, and since Oct. 15 falls on a weekend this year, taxpayers get an additional two days, making Oct. 17 the final day.

Taxpayers in some states will have even more time to get their 2015 tax returns in. The IRS said earlier this week that victims of Hurricane Matthew in certain counties in North Carolina would qualify for tax relief, allowing both businesses and individual taxpayers to push back some of the deadlines that they'd otherwise have to meet over the next several months. The IRS notice specifically included the Oct. 17 individual tax filing deadline for those who received extensions as included in the tax relief, and it gives affected taxpayers until March 15, 2017 to get their returns filed.

As of this writing, only 17 counties in North Carolina were included in the relief area, and no other affected states were yet included. However, the IRS said that it expected to see areas in other states, such as South Carolina, Georgia, and Florida, to be included as well. Damage assessments by the Federal Emergency Management Agency help determine whether tax relief is granted, and the timing of FEMA's work will likely determine when taxpayers in other areas get their tax relief.

Undoing a Roth IRA conversion

The other tax planning opportunity that goes away on Monday is the ability to undo a 2015 Roth IRA conversion. The process of what's known as recharacterization allows you to arrange with your financial institution to take the money that you had converted into a Roth IRA and return it to its original source, typically a traditional IRA. By doing so, you can undo the tax consequences of the conversion, reducing your taxable income for the 2015 tax year by the amount that you converted. For all intents and purposes, recharacterizing your Roth conversion makes it as though the conversion never happened.

The IRS gives you until the final filing deadline for the tax year in question to recharacterize a Roth, including extensions. That means this year's date is Oct. 17 for those who filed for an extension -- regardless of when they actually filed their 2015 tax return. Even if you got your 2015 return in on time back in April, as long as you filed for the extension, you gave yourself until Monday to do get your Roth do-over.

There are a couple of instances in which a Roth recharacterization makes a lot of sense. First, if the value of your converted Roth has gone down substantially since you converted, then undoing the conversion could help you cut your tax liability. By recharacterizing, waiting the required 30-day time period, and then doing another Roth conversion, you'll only pay taxes on the lower reduced value.

Also, many taxpayers find that they convert during a tax year in which they pay a high tax rate, only to see that rate drop the subsequent year. By recharacterizing a 2015 Roth conversion and then doing it in 2016 instead, you'll save on taxes if your marginal rate has gone down this year due to changes in your overall income.

Get it done

Procrastinators like the fact that they can get half a year beyond mid-April to get their taxes done. However, for most taxpayers, Monday is the last chance to avoid late-filing penalties that can add 5% to your tax bill each and every month that they're late. Moreover, if you've used a Roth conversion, it's worth taking a closer look to see if a last-minute recharacterization could help produce some valuable tax savings.