The islamic calendar is completely different from the western calendar. It commences when the prophet Muhammed emigrated from Mecca to Medina which corresponds to July 16 622, in the western, Gregorian calendar.

There are 12 lunar months of alternating 30 and 29 days, making a normal year of 354 days but time is divided into cycles 30 years long. In each cycle, 19 years have the regular 354 days but 11 years have an extra day each.

Ramadan is the ninth month in the Islamic calendar and is holy because it is said to be the time when the Quran was revealed to the people. Observation of Ramadam may well influence services available to travellers in islamic regions.

The hebraic calendar differs again. It is still in use today and begins at the day of creation which according to Jewish scholars occurred 3,760 years before the Christ. The week comprises 7 days, beginning with Saturday, the sabbath.

A normal year comprises 12 lunar months of either 29 or 30 days long. However, this produces a 'lunar year' which is 11 days shorter than a solar year. So time is divided into a 19 year cycle. Seven of these years are designated as leap years and in a leap year the month of Adar is repeated (ve-Adar).

In addition to the leap year cycle, the length of each year is slightly adjusted to meet a number of constraints called Dechiot. These small adjustments are made by selecting the length of the two months of Cheshvan and Kislev to be 29 or 30 days. The four constraints that determine the exact year length have to do with the exact timing of religious holidays in relation to the phase of the moon and the day of the week on which the holiday falls.

To western eyes both the islamic and hebraic calendars may seem bizarre but the western system is just as odd: why should February have 29 days instead of 28 but only once every 4 years ?

All these peculiarities arise because although the regular appearance of the new moon is an obvious occurrence on which to base a calendar and man has used this for at least 5000 years, the period of its orbit around the earth is not exactly divisible into the period of the earth's orbit around the sun. So, without correction, a calendar based only on the moon becomes more and more out of phase with seasons, climate and any religious ceremonies based on the position of the sun.

The earliest lunar calendar was devised by the Summerians, 5000 years ago. Later Babylonians subdivided months into weeks and weeks into 7 days. Captive Jews in Babylonia used the Babylonian 7 day week and so did the sun-worshipping Egyptians who went on to develop a 52 week solar calendar based on the 7 day week.

Julius Caesar corrected the Roman Calendar by making the year 46 BC one of 445 days to correct previous errors, thereafter every year was to have 365 days but every 4 years there was to be a leap year of 366 days by giving February one more day.

Caesar's rule creates 3 too many leap years in every 385 year period causing equinoxes and solstices to move away from their assigned calendar dates. Hence the correction by Pope Gregory 13th on October 4 1582 - the next day was made October 15 and the rule introduced that years ending 00 have to be divisible by 400 to be considered leap.

Many countries did not adopt the modern, western, Gregorian calendar for hundreds of years. It was not used in England until 1752 and was adopted in the Soviet Union only in 1918. Turkey did not take it until 1927.

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