It is thanks to Julius Caesar that today is February 29, instead of March 1. Some 2000 years ago, the Roman invader of Britain decided to reorganize the calendar, adding an extra day to every fourth year. With only one alteration we have followed this system ever since.

The problem facing all calendar organizers is that the earth takes 365 days, five hours, 48 minutes and 46 seconds to travel around the sun. The extra hours and minutes make an awkward fraction. Both the ancient Greeks and Chinese discovered this, and produced complicated calendars which had extra months included in every 19-year cycle.

But it was left to the Romans to produce a more practical solution - the leap day. By the year 46 B.C. the Roman festivals had got out of line with the seasons of the year, so Caesar's first act was to prolong the year [that particular year] to a total of 445 days. On the advice of a Greek astronomer, Sosigenes, he then [thereafter] started the leap year system.

Even so, the extra leap year didn't quite allow for the awkward fraction, since the so called Julian year was some 11 minutes 14 seconds longer than the natural year. In the course of centuries, the seasons moved slowly backwards toward an earlier date, so that by the 16th century the spring equinox (nowadays March 21) had moved backwards to March 11.

It was Pope Gregory XIII who made the next calendar adjustment. In 1582, he ruled that centurial years (those ending with double zeros) should be an exception to the normal leap year rule, that years divisible by four have the extra day. His papal bull established that centurial years would only have the extra day if they were divisible by 400. This is still followed, so that the year 1900 had no leap day, but the year 2000 will have its February 29.

Britain was late in following the papal adjustment since, being Protestant, it was disinclined to obey the Pope. It was not until 1752 that the Gregorian calendar came into operation here. By that time the disparity between our old time and the new calendar was 11 days, and thus September 2 was followed by the 14th in order to catch up. The change produced riots and cries of "Give us back our eleven days". (Russia didn't change over to the new system until about 170 years later.)

Even this system is not entirely correct. Every 10,000 years the Gregorian calendar becomes two days, 14 hours and 24 minutes in error. If we last long enough, adjustments will be necessary. One suggestion has been to make a further leap year refinement - excluding leap days from years divisible by 4000.

Another suggestion, particularly popular between the two world wars, was for a new world calendar. At the moment in Muslim countries it is the year 1396, and a year ends after every 12 lunar months. There are also at least 17 different calendars in India and Africa. But the problem is always to agree upon a new system which is acceptable to everybody.

Nobody is quite sure how February 29 came to be called leap day. It is possible because the extra day means that each date afterwards falls on the next weekday but one to that on which it fell in the preceding year. (In normal 365-day years, each date falls on the next weekday to the one it fell the year before.)

The leap year custom that women are allowed to propose is even more obscure. The earliest reference is a Scottish Act of 1228 which said: "Ordonit that during ye reign of her maist blisset Majestie Margaret, ilka maiden ladee of baith high and lowe estait shall hae liberte to bespoke ye man she likes - albeit he refuses to talk he shall be mulcted in ye sum ane pundis or less." Though 1228 was a leap year, there was no Queen Margaret at the time, which casts doubt on the Act's authenticity.

Another version maintains that St. Patrick originated the custom. Being told by St. Bridget that a nunnery was mutinying for the right to pop the question, the saint suggested they could have what they wanted every seventh year. St. Bridget then flung her arms about him, begging him to make it one year in four. "Bridget," exclaimed the saint, "squeeze me that way again and I'll give ye leap year, the longest of the lot." The wily St. Bridget then proposed on the spot, and St. Patrick had to give her a silk gown as a forfeit for his refusal.