Originally, the Persian calendar was lunar, following the 28-day cycle of the Moon. Since the year does not fit into an equal number of lunar cycles, however, the lunar calendar creates "seasonal drift" without a lot of alterations. This calendar was begun over 1000 years BCE. Khayyam was one of several scholars using astronomical observations to create a revised version. It was approved on 15 March 1079 by the Seljuk Sultan, Malik Shah I.
Khayyam and his team calculated the length of the year to be 365.24219858156 days; modern science puts it at 365.2422464 days. Some aspects of the new calendar:
- The year started within a day of March 21st, the vernal equinox
- Months were based on when the sun transited to a new sign of the zodiac, not 12:00AM
- Months could last from 29-32 days, and
- Months could change their length from year to year
That 4th point is because of the 2nd point. Months weren't given arbitrary numbers of days as in the West. The Jalāli calendar depended on strict astronomical data, not cultural numerical choices. Therefore, the 6th month of the year might have 30 days one year and 31 days the next, depending on when the sun passed across the line in the sky that separated the zodiacal signs. It also means that seasonal drift—the tendency of seasons to start and end on widely varying dates over time—never exceeded one day. Leap years were unnecessary.
Eventually, the varying length of the months was considered a liability. The calendar—still used in Iran and Afghanistan—was changed in 1925 in order to have a more regular look and to save the hassle of applying the results of constant astronomical observation.