Filing deadlines: 9/15 and 10/15

I've argued before that time is an artificial construct, a vain attempt by humans to control the inevitable.

Yeah, right. That's the IRS response to my esoteric theory.

Time is very real to the tax collector. Uncle Sam is a clock and calendar watcher, and two dates loom large for taxpayers.

Estimated taxes due 9/15: Monday, Sept. 15 is deadline day for estimated tax payment voucher #3.

This 1040-ES filing covers income that you earned in June, July and August, but from which no federal income taxes were withheld. This includes investment income you received during this period, either through regular mutual fund payouts or from the sale of an asset that produced a capital gain.

If your untaxed income during this period was from self-employment earnings, don't forget to factor in the added self-employment tax, too.

Even though this is payment number three, if the first time you received untaxed income was during the last few months, then go ahead and send in this payment now. While the IRS prefers to get four equal payments throughout the year, life doesn't always follow nice, delineated time frames.

The 1040-ES package has a worksheet to help you compute what you owe. If you use tax software, fire that back up to help you calculate your amount.

As for paying the amount due, if you're sending in a check, print out the voucher from the 1040-ES online form and get it in the mail by Monday.

If you're paying electronically and have set up an account with the IRS' Electronic Federal Tax Payment System, usually referred to by its acronym EFTPS, log in today to schedule that payment for Monday. (If you'd like to e-pay this way next time, this blog post has tips on setting up an EFTPS account.)

You can pay by credit card instead at Official Payments Corporation or Link2gov Corporation. Note that if you pay this way, you'll also be charged a processing fee of around 2.5 percent.

Whichever you choose, just make sure you meet Monday's deadline.

Or, if you have a job where withholding is already coming out of your paychecks, then fill out a new W-4 and have some extra taxes taken out for a couple of checks to cover your untaxed earnings. You can always change it back later to your regular withholding rate.

You can read more about estimated tax payments in this story.

Extension to file expiring soon: Then there are those of us who are paying 2008 estimated taxes but have yet to file our 2007 IRS forms.

We've got just over a month, 32 days to be precise, to wrap up last year's tax tasks. You can keep track of the countdown to the Oct. 15 final Form 1040 filing deadline via the countdown clock there in the right column of the ol' blog.

Don't put this off until late on 10/15. You need to give yourself some cushion to get your forms filed as the October date is the absolute final date to finish up your 2007 taxes. Of course you paid a good estimate of your eventual tax bill when you got the extension back in April, so you've done most of the riling work anyway. Don't let that effort go to waste.

Remember, if the amount you sent in with your extension request comes up short and you don't get your return and the balance in by Oct. 15, the IRS can whack you with a penalty for nonfiling as well as for paying late.