Are you waiting until the last possible minute to prepare your 1999 tax return? If you are, be sure your online tax preparation site is prepared to accept your return as the clock ticks down to midnight on Monday.

Many sites require your return be submitted well before midnight on Monday to guarantee it reaches the Internal Revenue Service on time. But none of these sites tells you so.

So we've compiled a list of drop-dead deadlines for you.

Just because you hit "send" on a tax preparation site doesn't mean your return has been accepted by the IRS. It can take up to 48 hours to hear back from Uncle Sam and get a DCN, or declaration control number, which indicates your return was accepted. That number goes on Form 8453-OL -- U.S. Individual Income Tax Declaration For On-line Services Electronic Filing, which must be mailed in the next working day after the IRS accepts your return. (If you've filed online in years past, you probably can skip this step.)

But if your return is not accepted for whatever reason, it is not considered filed yet. It does not matter that you made a valid attempt. So factor that into your last-minute planning.

Keep in mind that none of these sites will shut down at midnight on Monday. They're just not promising that last-minute returns will get to the IRS before the deadline. Maybe they will, maybe they won't. Feel free to take a chance at 11:58 p.m. It's your risk.

Here's a day-by-day and hour-by-hour breakdown of the deadlines. (Check out our online tax-filing guide for more detail on the sites discussed below.)

Unless otherwise noted, all times indicated are your local time.

Friday April 14

hdvest.com, the all-free-for-all-people tax preparation Web site, recommends that you file no later than midnight on April 14. If you've started a return here, but haven't filed it yet, do so soon. It's not guaranteed to reach the IRS by the deadline, but it still might.

CompleteTax also set a Friday midnight deadline. This site requires you to fill out an electronic questionnaire and email it back to the company. Its system then turns your responses into a tax return, but the turnaround time is 24 hours.

Monday April 17

Noon

You have until noon on Monday to use the live-preparer option on e1040.com. If you make that deadline, "we'll get 'em done," promises e1040.com Vice President Kathy Travis.

The live-preparer option requires customers to fill in a questionnaire on the site that is immediately sent to a live preparer. The preparer promises to contact you within 20 minutes (during business hours) with any follow-up questions.

e1040 also has an automated e-filing program (federal only) you can use until midnight.

6 p.m.

If you're a pro and want to just input your numbers directly onto IRS forms, esmart TAX might be your answer. The forms are Microsoft Word files that calculate your tax bill. Just make sure you file by 6 p.m. for guaranteed on-time delivery to the IRS.

10 p.m.

Using TaxAct? Get your return in by 10 p.m. Central Daylight Time to guarantee it reaches the IRS before the deadline.

11 p.m.

If you're using TaxCut or hrblock.com, get your return in by 11 p.m. "Then we'll guarantee a timely file," says Linda McDougall, vice president of communications for H&R Block (HRB:NYSE - news). "And if a problem arises, we'll cover the penalty."

Midnight

TurboTax, the king daddy of tax-preparation software, says you can file until midnight and your return will be considered on-time, says Jennifer Roberts of Intuit, maker of the software. Because 75% of all electronically filed returns come from TurboTax, the IRS cuts the company some extra slack.

Beyond Midnight

On Monday, I'll give you the lowdown on filing for an extension and paying off Uncle Sam.