It was the custom of the Persians not to begin a march before sunrise. When the day was already bright, the signal was given from the king's tent with the horn; above the tent, from which it might be seen by all, there gleamed an image of the sun enclosed in crystal. Now the order of march was as follows. In front on silver altars was carried the fire which they called sacred and eternal. Next came the Magi, chanting their traditional hymn. These were followed by three hundred and sixty five young men clad in purple robes, equal in number to the days of the whole year; for the Persians also divided the year into that number of days.
Quintus Curtius Rufus: History of Alexander, III, iii (circa 35 c.e.)
The modern Persian calendar, adopted in 1925, is a solar calendar based on the Jalālī calendar designed in the eleventh century by a committee of astronomers, including a young Omar Khayyām, the noted Persian mathematician, astronomer, and poet. The Jalālī calendar had 12 months of 30 days each, followed by a 5-day period (6 in leap years), just like the Coptic and Ethiopic calendars described in Chapter 4. In addition to the Jalālī calendar, the Zoroastrian calendar, whose structure is described in Section 1.11, was also used historically in Persia. The lengthy history of Persian calendars is discussed in , , and ;  gives a briefer history, together with tables and computational rules for the arithmetic form of the calendar to be discussed in Section 15.3. A calendar identical to the modern Persian calendar, but with different month names, was adopted in Afghanistan in 1957.
Epochæ celebriores, astronomis, historicis, chronologis, Chataiorvm, Syro- Græcorvm Arabvm, Persarvm, Chorasmiorvm, usitatæ [Famous epochs customarily in use by astronomers, historians, chronologists, Hittites, Syrian-Greeks, Arabs, Persians, and Chorasmians]Title of John Greaves’ Latin/Persian edition (1650) of a work by the fourteenth-century Persian astronomer Ulugh Beg, grandson of Tamerlane
The epoch of the modern Persian calendar is the date of the vernal equinox prior to the epoch of the Islamic calendar; that is, 1 a.p.