It never fails. Whenever you leap into leap years, you leap into numbers such as 1, 366 and 29.

But with February 2008's official leap day just five days away, we thought we'd introduce you to a few other and perhaps less familiar numbers that you should know, according to Thomas O'Brian, the chief of the Time & Frequency Division at Boulder's National Institute of Standards and Technology. This help explains why, every four years or so, it's so important to add one more day to the year's dreariest month.

"If we didn't have leap years, we'd start having winter weather in June," O'Brian said. "It would take hundreds of years, but the calendar would get way out of whack."

Among some numbers to noodle:

365

You know this as the number of days in the typical calendar year, based on the number of days it takes Earth to travel once around the sun. Oops. It's not very accurate.

365 1/4

This is closer to what it really takes. Egyptian, Babylonian and Sumerian astronomers knew as much thousands of years ago.

45 B.C.

The year that the Julian calendar was introduced by the Roman Emperor Julius Caesar (whose name, by the way, gives us the month "July." We get "August" from Augustus Caesar, who came later). It didn't take long for society to notice the ill effect of all uncounted quarter-days. Feasts and celebrations seemed to get later and later. So the Julian calendar added a "leap day" to the end of February every four years to even things out. That didn't work right either.

365.242216

The real number of days it takes for Earth to travel once around the sun. It wasn't exactly 365 1/4, but a little less. So the Julian calendar was actually adding too much time.

11 MIN., 14 SEC.

The Julian calendar was off by this much. Over the centuries this extra time started to add up. Again, the calendars were off.

1582

Greet the new Gregorian calendar, the one we use today. It was introduced under the reign of Pope Gregory XIII. Instead of adding a "leap day" every four years, the Gregorian calendar came up a variation on the four-year rule. Yes, every fourth year would be a leap year. But a century year — such as 1600, 1700, 1800 and so on — would not be a leap year unless it was divisible by ...

400

Yes, that's right. Unless the century can be divided evenly by 400, it is not a leap year. So 2000 was a leap year. But 2100 will not be. Although it sounds arbitrary, it works extremely well.

365.2425

This is the average length of the calendar year using the Gregorian reform. "It's not exact," O'Brian said. "But it's so close it would be thousands of years before we would have to worry about making an adjustment to the Gregorian calendar." Oh, and as to why we add the day to February? The answer to that question is a little less precise.

Both January and February are late additions to the Roman calendar, which used to have 10 months running from March to December (October referring to "eight," November to "nine," December to "10."). February is now our second month, but it used to be the 12th and, as such, already had fewer days than the other months. When it came time to add a leap day, it seemed right to add it at the end.

Then there's this notion: "The story goes — and it's just a story as far as we know — that when Julius Caesar was looking at the length of months, he wanted the month that honored him, July, to be a long month," O'Brian said. "So he took a day away from February to make his month 31 days long. Then, later, Augustus Caesar also took a day away from February. At one point February had more days, but it lost a couple of days to the Roman emperors."