Persian calendar Medieval era: Jalali calendar In 1079 CE, the Jalal Al Din Shah Seljuqi ordered to change the Islamic Calendar -that is based on the lunar system- to the calendar of Omar Khayyam; this new calendar was called the Jalali Calendar. Khayyam did an effort for 8 years in Isfahan, that was capital of Iran in the time of the Seljuq dynasty. In this new system Khayyam planned his calendar so