The Internal Revenue Service is all about numbers, and that includes dates on calendars. If you don't file your return on time and if you owe tax, you'll face penalties and interest.

Everyone knows about April 15, the day tax returns are due. But as countless procrastinators can attest, it's when your return must be en route to the IRS, not when the agency actually has to have your 1040 in hand.

The IRS considers a paper return filed on time if it is mailed in a properly addressed envelope, has enough postage and is postmarked by the due date. The official term, per Section 7502 of the Internal Revenue Code, is "timely filed." The tax code also notes that the date on a U.S. Postal Service receipt for certified or registered mail also qualifies.

As long as the tax mailing has the appropriate USPS cancellations showing it was mailed on or before the due date, the return is considered on time no matter when the IRS actually receives it.

Private delivery accepted
What if you're more comfortable using a private delivery service? That's OK with the IRS and has been since 1997.

In these cases, the IRS considers a timely filed postmark to be the date a private delivery service records in its database that it accepted the tax document or marks that date on the mailing label.

The IRS has designated three companies as authorized private delivery services: DHL Express, Federal Express and United Parcel Service. If you choose to use one of them, check with its local office as to the company's requirements and pick-up deadlines, as well as the specific private delivery services the IRS accepts.

Filing on time electronically
Electronic filing is popular in part because it enables you to send your 1040 from a personal computer with a simple push of the "enter" button.

E-filing also provides an almost immediate acknowledgment that the return is on its way to Uncle Sam. This notification from tax software companies and other electronic return originators is your official electronic postmark.

Technically, an e-filed return is not considered filed until the IRS acknowledges that it has been received and accepted for processing. However, if the electronic transmission is successful and completed on or shortly before the due date, the IRS considers it as timely filed.

But don't wait until 11:59 p.m. April 15 to e-file. Although such a last-minute electronic timestamp is acceptable to the IRS, you run the risk that your online connection might be bogged down by other late e-filers. Worse, your computer could crash, not only causing you to miss the filing deadline but also destroying your entire return.

Pushing the deadline
Sometimes April 15 comes and goes without any tax consequence. That's the case when it falls on a weekend or federal holiday. In these instances, the next business day becomes the official tax-filing deadline.

The IRS also allows you, if you file the proper paperwork, six more months to complete your Form 1040. Submit Form 4868 by the regular April due date, and your filing deadline becomes Oct. 15, or if that date falls on a weekend or federal holiday, the next business day.