Editing Iranian calendars (section)

==Medieval era: Jalali calendar==
{{Main|Jalali calendar}}

In AD 1079, by the order of the Jalal Al-Din Shah [[Seljuq dynasty|Seljuq]]i, the Islamic Calendar (which was and is based on the lunar system) was replaced in Persia by the calendar of [[Omar Khayyam]] and was called the Jalali Calendar. Khayyam and his team had worked 8 years in [[Isfahan]], the capital of Iran during the [[Seljuq dynasty]]. The research and creation of the Khayyam calendar was financially supported by Jalal Al din Shah. Khayyam designed his calendar in which the beginning of the new year, season and month are aligned and he named the first day of the spring and the new year to be Norooz (also spelled [[Nowruz]]). Before Khayyam's calendar, Norooz was not a fixed day and each year could fall in late winter or early spring.

From 15 March 1079, when the calendar had slipped a further eighteen days, the ''araji'' calendar was reformed by repeating the first eighteen days of Frawardin. Thus 14 March was 18 Frawardin ''qadimi'' (old) or ''farsi'' (Persian) and 15 March was 1 Frawardin ''jalali'' or ''maleki'' (royal). This new calendar was astronomically calculated, so that it did not have ''epagemonai'' – the months began when the sun entered a new sign of the zodiac.

About 120 years after the reform of AD 1006, when the vernal equinox was starting to fall in Ardawahisht, Zoroastrians made it again coincide with ''nowruz'' by adding a second Spandarmad. This ''Shensai'' calendar was a month behind the ''qadimi'' still used in Persia, being used only by the Zoroastrians in India, the Parsees. On 6 June 1745 ([[Old Style and New Style dates|Old Style]]) some [[Parsee]]s re-adopted the ''qadimi'' calendar, and in 1906 some adopted the ''Fasli'' calendar in which 1 Frawardin was equated with 21 March, so that there was a sixth epagomenal day every four years.   In 1911 the ''jalali'' calendar became the official national calendar of Persia. Some Zoroastrians in Persia now use the ''Fasli'' calendar, having begun changing to it in 1930.