The Top 40

Like almost everything else in their wretched religion, the Muslims did not get the idea of a calendar right.

Their sacred calendar consists of a cycle of 12 lunar months. Since a lunar month is about 29.5 days, a "Muslim year" is 354 days, which is 11 days short of a year.

As a consequence of the lunar cycle, the date of Ramadan is 11 days earlier in a real calendar every year. Furthermore, 33 Muslim years are about 32 years.

"Muslim years" are counted since the Hijra(Mohammed's flight to Medina on 16 July 622 C.E.). It is currently "Muslim year" 1431, even though only 1387 years have passed since then. The "Muslim year" is useless for the purposes that years are used by rational men.

There are other calendar systems that use lunar moths, such as the Hebrew and Chinese systems. But, they use intercalations of lunar months (7 in 19 years) to keep the year and the month synchronized. The system of intercalations, the Metonic cycle, has been known since antiquity and was readily available in Arabia in the 7th century.

More info:
http://www.tondering.dk/claus/calendar.html