M uslims use a purely lunar calendar. It was adopted in A.D. 632, two months before the death of Mohammed. At that time, Muslims did not maintain a count of years. That's a little more than eleven days short of a solar year. New Year's Day, the first of Muharram, occurs eleven days earlier each year than it had the year before.
Islamic months begin at sunset on the day that the lunar crescent is actually sighted.
Religious doctrine requires that visual sighting is necessary to determine the start of a month, even though the date a new crescent is likely to be visible can be accurately predicted. Visibility of a new crescent depends on a large number of factors including weather conditions, atmospheric pollution and whether or not optical aids are used. Because of this, some members of the religion believe they should use predicted dates of new crescents rather than actual sightings. However, many Muslim scholars support using calculations only to negate erroneous sightings, not to replace correct sighting.