In a perfect world, perhaps, everything would come out even. The Earth would move around the sun in exactly 360 days. The calendar would be divided into 12 months of 30 days each, months that would exactly coincide with the time it takes for the moon to rotate around the Earth.

But that's not the way it is.

In our world, the Earth takes 365 1/4 days to complete a rotation around the sun. The lunar cycle is 29 1/2 days long. Those extra days and fractions may not seem like much, but they have the power to wreak havoc with calendars and have led to what author J.B. Priestly calls "a long struggle to make a tidy job out of rather untidy natural units of time."

Times and seasons have always been important. People needed to know when to plant crops, watch for floods and honor their various gods. But coming up with a workable system was not always easy.

For example, the ancient Egyptians were among the first to use a solar calendar, which they adopted around 4000 B.C. The year had 365 days and used 12 30-day months, with five-day weeks, and five days of festivals. That came close to the solar year but was off just enough over time to make a difference.

The Babylonian calendar was composed of alternating 29-day and 30-day months, which added up to a 354-day year. When the calendar began to drift too far from seasonal events, another month was added. To further adjust things, three months were added every eight years.

The earliest Roman calendar, developed about 700 B.C., had 304 days with 10 months. Every second year, a short month of 22 or 23 days was added to square things with the solar year. Eventually two more months were added at the end of the year, to increase it to 354 days.

This process of adding more time was called "intercalating," something frowned on by some people who didn't like the constant tinkering with time. Plus by the time of Julius Caesar, the calendar was about three months ahead of the seasonal year.

By Caesar's time, people had finally realized that the solar year was actually 365 1/4 days long — which, when you think about it, is quite a remarkable achievement. In 46 B.C., Julius Caesar came up with a calendar that would address the problem of the extra fourth of a day. Every four years an extra day would be added. Leap year was born.