Time management: Need more than 24 hours in your day? Blame the Egyptians, writes Brendan McWilliams

"Time has no divisions to mark its passage," says Thomas Mann in The Magic Mountain. "There is never a thunderstorm or a blare of trumpets to announce the beginning of a new month or a new year, and even when a new century begins, it is only we mortals who ring bells and fire off pistols."

Nonetheless, some of the milestones we use to measure time's passage are rooted in the natural world. The length of both the year and the month, for example, are related to celestial motions. A year is the duration of the Earth's journey around the sun, and the month - rather imprecisely, as it happens - relates to the length of time it takes the moon to orbit Earth. But the further subdivisions of these entities are somewhat arbitrary.

The seven-day week appears to have originated with the Chaldeans in Babylon around 500 BC. Many historians ascribe it to the mystic nature of the number seven in many eastern cultures, but others believe the seven-day week may also have a celestial connection, based on the number of planets known in the ancient world. This latter relationship became more firmly established when the seven-day week was adopted by the Romans in the fourth century AD, and the different days were named after the Roman gods associated with these seven planets.

The origins of the 24-hour day, and the reasons for this particular number, are lost in the mists of time. The early Romans divided their day into only three parts - morning, afternoon and night - each bounded by two of the three phenomena of sunset, dawn and noon. Dawn and sunset were impossible to miss, but noon was more difficult; it was the moment of the shortest shadows of the day, or for the Romans the time of the arrival of the sun between the Rostra and a spot called Graeco-stasis, the place where the Greek ambassador was wont to stand.

In due course, however, the notion of a day divided into 12 hours of daylight, 10 of night and two of twilight, was borrowed from the Egyptians, and the number 24 became established. But throughout the mediaeval period, all hours were not of equal length. Twilight was abandoned, and the two daily periods of light and darkness were each divided into 12 equal parts called temporal, or "unequal", hours, with the inconvenient consequence that the length of an hour varied from day to night, and also with the changing seasons.

This awkward arrangement is believed to have been revised at the suggestion of one Abul Hassan, a 13th century Arabian mathematician who defined the hour as a fixed period based on "a twelfth part of the daylight at the equinoxes".

The origins of our minute and second also go back to the Babylonians, being based on the system they used for astronomical calculations. They employed a sexagesimal system that had the number 60, rather than 10, as the corner-stone of its construction. During the mediaeval period, a bewildering variety of methods were used to sub-divide the hour but, by the late Middle Ages, the arrangement of minutae primae, or "first very small parts", and minutae secundae, "second minute parts", had come into general use, adapted from the geometrical system for measuring degrees of arc. These units are with us still as minutes and seconds.

Over the centuries, efforts have been made to improve on these arrangements. Following the French Revolution, for example, republicans were irked by the traditional Gregorian calendar, redolent as it was of Gods and Roman Emperors, and in 1792 they unveiled a radical revision.

The 12 new months were uniformly 30 days in length, with names related to the agricultural activity most common at the time of year, or to the weather from which the month might take its character. Since the 12 months together amounted to only 360 days, the extra five or six required to keep in step with nature were added here and there, and designated festivals or public holidays.

The seven-day week was abandoned. Each month was divided instead into three weeks of 10 days each, with the last day of each decade being a rest day. The days themselves were subdivided into 10 "hours", each of which turned out to be something over two old hours in length.

The new calendar, however, never caught on. It was widely used for a time in conducting the affairs of State, but the ordinary people found it hard to change from the Gregorian system they were accustomed to. One of their complaints was that they now had only one day off in 10, instead of one in seven. The experiment was finally abandoned by Napoleon at the beginning of 1806.

There have been more recent reforms of this kind, as revolutionary in their own way. In 1989, for whatever reason, Leeds City Council in Yorkshire is said to have decreed that metric time should be used in all council business. The 60-minute hour was abolished in the borough, and the hour was divided instead into 10 units of six minutes each.

Thus, for example, 9.06 a.m. became 9.1 in Leeds New Time, and if you had an appointment with the mayor at 14.25, his worship would expect you to arrive at what we would call 2.15 p.m. precisely. The extent, if any, to which Leeds New Time is still in use, we do not know.