In the Julian calendar, the first of the year was also moved from March to January, so the year as we know it came into being. The year had 12 months of 30 or 31 days, except for February, which had 28 days. February was originally the last month of the year, which was why it was shorter and why it got the extra day. The first year under the Julian calendar was stretched to 445 days to get things back on cycle. After that, it seems to work well, at least for a few centuries.

But the Julian calendar had a small problem; it was 11 minutes too long. That may not seem like much, but by the 16th century, seasonal days were a week off.

This posed a problem for the Catholic Church, which set the date of Easter depending on the first full moon after the vernal equinox. If things kept going the way they were, Easter could end up far from spring.

So, Pope Gregory XIII came up with a reform that would provide a solution: Every year divisible by 4 would be a leap year. However, the calendar would not have a leap year in the century years that were divisible by 100, unless that year was also divisible by 400, then it would be a leap year.

Just how they came up with this formula is uncertain, but it works pretty well.

It means that 1800, 1900, 2100, 2200, 2300 and 2500 are not leap years, but 2000 and 2400 are. The year 2000 was the first time in many countries that the third part of the rule came into play. Over the years, it has taken care of the pesky extra minutes quite nicely, except for a few seconds that have to be adjusted now and then.

The Gregorian calendar was adopted by Catholic countries in 1582. It took the Protestants awhile to decide it had scientific and practical, rather than just religious, value. In England and America, 11 days were added to adjust things in 1752, and the Gregorian calendar became the standard. But some countries did not adopt it until after World War I.

Today, Friday, Feb. 29, 2008, is leap day. The last time leap day was on a Friday was 1980; the next time will be 2036. It takes 28 years for the same day to come up again.

In the 2,054 years since Julius Caesar first proposed the idea, leap year and leap day have become an integral part of the calendar. Yet, it's fun, every now and then, to stop and think where we'd be without them. It would be a very untidy world, indeed.

Year of the Frog

• Kickoff day for the Year of the Frog. Sponsored by the Association of Zoos and Aquariums, the campaign is designed to educate people about amphibian conservation. According to the association, one-third of all amphibians on the planet are threatened with extinction, due to loss of habitat, as well as infectious diseases.

Many zoos and aquariums will host special activities today and throughout the year. To learn more, go to www.aza.org/YearoftheFrog/.

You can also celebrate the day by learning some fun frog facts. For example: