One of the first things you notice if you follow taxes is that the tax collector operates on a different timetable than most of us.

Sure, tax years in most cases start Jan. 1 and end Dec. 31. But in between, the calendar tends to get jumbled.

Tip reporting is due on the 10th of each month. Businesses face a variety of different due dates throughout the year. And the middle of the month is the biggest date for individual taxpayers.

It’s usually April 15 that we focus on. But if you make money that doesn’t have taxes withheld from it, then you have to worry not only about mid-April, but also three other 15th deadlines. They are the June, September and January due dates for estimated taxes.

And the third payment for 2012 estimated taxes is due Monday, Sept. 17.

Wait. Didn’t I just say the 15th was the crucial day? Yes, except — and here’s where the tax man plays with the calendar again — when the 15th falls on a weekend or a federal holiday.

Sept. 15 falls on Saturday this year, so that means you get two extra days to send in your 1040-ES for the third quarter of 2012 earnings. This coming payment covers money made during June, July and August that you didn’t pay taxes on.

As I note in my Bankrate story on estimated taxes (which includes a table showing the four estimated deadlines and the earnings periods they cover), a lot of taxpayers face these extra tax filings. They get untaxed money from self-employment, investments, alimony and, if they’re lucky, prizes they’ve won.

Since our tax system is set up on a pay-as-you-earn basis, if you don’t make estimated tax payments on that money, you could end up owing not only the actual taxes due, but also penalties and interest for not paying them on time.

So take note of the Internal Revenue Service calendar, and don’t miss Monday’s estimated tax payment deadline.

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