The Hijri calendar is not to be confused with the Solar Hijri calendar used in Iran and Afghanistan. The Islamic calendar is a lunar calendar whose time reckoning is tied to the Moon phases.Each month lasts for a full lunation, which is the time span from one New Moon to the next.Because the time of moonset at a location depends on its longitude, a new month and key religious rituals like the Ramadan fast may begin a day earlier in, for example, West African Muslim countries than in Indonesia or Malaysia.The Islamic calendar has 12 months with 29 or 30 days.Some countries and Muslim communities now use modified versions of the traditional calendar that are designed to make the timing of Islamic months and observances easier to predict.
If no sighting is made, a 30th day is added to the current month, which is then followed by the first day of the subsequent month. The Persian and Islamic calendar systems are otherwise unrelated.
Unlike other calendar systems that use leap days or leap months to synchronize the calendar with the solar year, the Islamic calendar is completely detached from astronomical seasons marked by the equinoxes and solstices.
An Islamic year consistently falls about 11 days short of the solar year.
Even though the Islamic calendar year does not match up with the length of a solar year, it does not feature a correction mechanism like leap days in the Gregorian calendar to make up for the deviation.
For each year that passes, Islamic dates fall on earlier dates in the Gregorian calendar.