Hijri and Gregorian Dates

The Islamic (Hijri) calendar is a lunar calendar. Each new month starts from sunset on the evening when the new moon appears. The lunar year consists of 12 lunar months and is therefore about 10 to 11 days shorter than the solar year used in the West in the Gregorian calendar. Days run from sunset to sunset, not midnight to midnight.

The Islamic (Hijri) calendar started from Mohamed’s migration from Mecca to Medina in 620 A.D. As a result of this, as an example, the Islamic (Hijri) year 1400 A.H. corresponds within a few months to the Gregorian year 1980 A.D.

Arab Muslim countries produce calendars based on projections of when the new moon will appear at sunset for each month, but these are subject to change in accordance with actual sightings. In particular the month of Ramadan, when Muslims fast, starts and ends in line with actual moon sightings. For the festival which follows the month of Ramadan, moon sighting committees are established in each country to confirm the sighting in accordance with special rules for the acceptance of witness statements of the sighting of the new moon.

Because of the rotation of the earth and the revolution of the moon round the earth the astronomically calculated “new moon” can occur at any time according to say Greenwich Mean Time. Hence there may be a new moon visible at sunset in say Saudi Arabia a few hours after the calculated time, while at sunset a few hours earlier in a more eastern country such as Indonesia the sun has set before the time of the calculated new moon. In other words a new moon might be theoretically and actually visible at one sunset in Saudi Arabia but not till the next evening in another country such as Indonesia.

Hence Islamic (Hijri) dates do not necessarily correspond to the same Gregorian date in all Islamic countries.

Furthermore the moon is very close behind the sun at sunset at a new moon and may be difficult to see, and sky conditions can affect visibility. This all adds to the difficulty of precisely equating Islamic (Hijri) dates to Gregorian dates. In practice the error for any date is unlikely to be more than two or three days and is usually less.

Similarly it is not possible to predict in advance on which day of the week a particular Islamic (Hijri) date will fall.

Diaries are produced with Gregorian and Islamic (Hijri) dates based on astronomical predictions, but these can turn out not to be correct.

For all these reasons a number of Islamic countries use Gregorian dates for practical purposes.

When requested to convert dates I use a computer conversion program written by my son. This has proved accurate within the limitations of the inherent variations explained above.

Libyan dates (stated to be “after the death of the prophet” correspond to Gregorian dates except that the year is 632 less than the Gregorian year and the names of the months are special to Libya.

© Philip Gordon 2003

Return to home page