Leap year is serving up its bonus day today, Feb. 29, 2012. Known as leap day, Feb. 29 is an added day that seems to magically show up on the calendar every four years. You might be wondering, what is leap year and leap day, and why do we mess with the calendar?

We called the super-smart folks over at the US National Institute of Standards and Technology for some answers. (Feel free to crib from it and play the know-it-all when talking to friends today.)

Leap year and leap day are little more than astronomical catch-up days, said John Lowe, who heads the time and frequency services group at the institute -- the federal agency responsible for, among other things, establishing the official time in the United States. That's the time that virtually all other clocks -- including the clock on your cellphone and computer -- are set against. (That kind of makes Lowe the country's official timekeeper. But you can't blame him if you're running late.)

Clocks and calendars and days are all universally accepted methods we use to track time in the universe, but these human-made methods only take us so far. Earth doesn't rotate around the sun in precisely 365 days. It actually takes about 365.25 days.

"We end up with about a quarter-day error every year," Lowe said. "So you throw in an extra day every four years to make up for that."

There are some exceptions to this every-four-years adjustment, however, because Earth doesn't rotate around the sun in precisely 365.25 days. As a result, some would-be leap years skip leap day entirely, as took place in the year 2000. The next time we skip a leap day will be 2100.

If we didn't do that, Lowe said, we'd eventually find ourselves celebrating Easter in the dead of winter. That's what led to the calendar reconfigurations to accommodate leap year and leap day.

So why does February get the extra day? Why not December or June or October?

Leap day comes in February because it used to be that February -- and not December -- marked the end of the year. So the extra day was tacked on to the end of the year, Lowe said.

"Everybody just kind of assumes that we've got the calendar and that's the way the calendar has always been, but that's not true," Lowe said. "It has changed dramatically throughout history."

While we had Lowe on the phone, we couldn't help but ask: Is he ever late? He laughed. "Because of my line of work, I have to be punctual. Or else people will never let me hear the end of it."