Tabular Islamic calendar

The Tabular Islamic calendar is a variation of the Islamic calendar. It has the same numbering of years and months, but the months are determined by rules rather than by observing the crescent moon directly. The calendar was developed by early Muslim astronomers of the second hijra century (the 8th century of the Common Era). They wanted to be able to predict the positions of the moon, sun, and planets. It is now used by historians to convert an Islamic date into a Western calendar when no other information (like the day of the week) is available. Its calendar era is the Hijri year and its epoch (year 1) is the date of the Hijra (622 in the Common Era or Gregorian calendar).
Uses.
It is used by some Muslims in everyday life, particularly in Ismaili and Shi'a communities, who believe that this calendar was developed by Ali. They believe that when Ali drew up this calendar, the previous events of the earlier prophets also fell into line with this calendar. It is their belief that all Fatimid Imams and their Da'is have followed this tradition.
Days and months.
Each year has 12 months. The odd numbered months have 30 days and the even numbered months have 29 days, except in a leap year when the 12th and final month Dhu al-Hijjah has 30 days.