The Islamic Calendar

The Islamic calendar (or Hijri calendar) is a lunar calendar. It contains 12 months that are based on the motion of the moon, and because 12 lunar months is 12 x 29.53=354.36 days, the Islamic calendar is consistently shorter (11 Days) than a solar year, and therefore it shifts with respect to the Gregorian calendar.

The Islamic calendar is the official calendar in countries around the Gulf, especially Saudi Arabia. But other Muslim countries use the Gregorian calendar for civil purposes and turn to the Islamic calendar for religious purposes.

What does an Islamic year look like?

What does the Quran say about measurement of time?

[Quran - 9.36] Surely the number of months with Allah is twelve months in Allah's ordinance since the day when He created the heavens and the earth..

[Quran - 31:29] Seest thou not that Allah merges Night into Day and he merges Day into Night; that He has subjected the sun, and the moon (to his Law), each running its course for a term appointed; and that Allah is well-acquainted with all that ye do?

[Quran - 103: 1..3] By (the Token of) Time (through the ages),-  Verily Man is in loss, -  Except such as have Faith, and do righteous deeds, and (join together) in the mutual teaching of Truth, and of Patience and Constancy.

How is an Islamic month determined:

Each month starts when the lunar crescent is first seen after a new moon.

Although new moons may be calculated quite precisely, the actual visibility of the crescent is difficult to predict. It depends on factors such as weather, the optical properties of the atmosphere, and the location of the observer. Therefore in some cases it may be difficult to give accurate information in advance about when a new month will start.

Furthermore, in some areas Muslims depend on a local sighting of the moon, whereas in other areas a universal sighting is accepted. (i.e. if a new crescent is seen anywhere in the world it is accepted for communities the world over). Both are valid Islamic practices, but they may lead to different starting days for the months.

Islamic Calendar for North America:
Source: Moonsighting.com

2003 Calendar with Islamic Dates	1424 Calendar with Gregorian Dates
2004 Calendar with Islamic Dates	1425 Calendar with Gregorian Dates
2005 Calendar with Islamic Dates	1426 Calendar with Gregorian Dates

Rules for the Islamic Calendar in Different Countries

The Islamic calendar is based on visibility of the crescent Moon.  We know perfectly well where the Moon and the Sun are at any given time, but how light must the Moon be and how dark must the sky be before we can see the crescent? And what if the weather is bad?

In spite of these difficulties, some Muslim communities base their calendar on actual Moon sightings. This is done in India, Pakistan, and Bangladesh.

In some parts of the world there is a growing trend to determine the start of the new lunar month according to the minimum criteria required for sighting the new crescent. This is .. the age of the new moon must be at least 8.5 hours to 15.5 hours and the separation between the sun and the moon must be at least 11 degrees, to be able to be seen with the naked eye. 

In Saudi Arabia and most of the Gulf countries they have simplified the calendar. They start the lunar month if the Moon sets after the Sun on the 29th day of the previous month, as seen from Mecca.

How does one count years?

Years are counted since the Hijra, that is, Prophet Mohammed's (Peace be upon him) migration to Medina, which is assumed to have taken place 16 July C.E. 622 (Julian calendar). On that date AH 1 started (AH = Anno Hegirae = year of the Hijra).

Note that although only 2002 - 622 = 1380 years have passed in the Christian calendar, 1423 years have passed in the Islamic calendar, because its year is consistently shorter (by about 11 days) than the tropical year used by the Christian calendar.

When will the Islamic calendar overtake the Gregorian calendar?

As the year in the Islamic calendar is about 11 days shorter than the year in the Christian calendar, the Islamic years are slowly gaining in on the Christian years. But it will be many years before the two coincide. The 1st day of the 5th month of C.E. 20,874 in the Gregorian calendar will also be (approximately) the 1st day of the 5th month of AH 20,874 of the Islamic calendar!

Further information & and some more links you might be interested in

• Moonsighting.com - provides moon visibility curves for the current year and next year.
• U.S. Naval Observatory - lists new moon birth times for the whole year. NB - the crescent moon will not be visible until at least 15 hours after the birth of the new moon.
• IslamiCity Prayer Times
• IslamiCity Gregorian/Hijri Converter