One of the longest chronological records in human history, the Iranian calendar has been modified many times for administrative purposes.
The most influential person in laying the frameworks for the calendar and its precision was the 11th century Persian polymath, Omar Khayyam.
The Iranian New Year begins at the midnight nearest to the instant of the northern spring equinox, as determined by astronomic calculations for the meridian (52.5°E).
The first fully preserved calendar is that of the Achaemenids, a royal dynasty of the 5th century BC who gave rise to Zoroastrianism.
The sun has always been a religious and divine symbol in Iranian culture and is the origin of the folklore regarding Cyrus the Great.
[2] Old Persian inscriptions and tablets indicate that early Iranians used a 360-day calendar based on the solar observation directly and modified for their beliefs.
The unified Achaemenid Empire required a distinctive Iranian calendar, and one was devised in Egyptian tradition, with 12 months of 30 days, each dedicated to a yazata (Eyzad), and four divisions resembling the Semitic week.
Thirteen days were named after Fire, Water, Sun, Moon, Tiri and Geush Urvan (the soul of all animals), Mithra, Sraosha (Soroush, yazata of prayer), Rashnu (the Judge), Fravashi, Bahram (yazata of victory), Raman (Ramesh meaning peace), and Vata, the divinity of the wind.
The remaining four were dedicated to Asman (lord of sky or Heaven), Zam (earth), Manthra Spenta (the Bounteous Sacred Word) and Anaghra Raocha (the 'Endless Light' of paradise).
It fixed the pantheon of major divinities, and also ensured that their names were uttered often, since at every Zoroastrian act of worship the yazatas of both day and month were invoked.
In 538 BC Cyrus the Great (uncertain if he was a Zoroastrian) conquered Babylon and the Babylonian luni-solar calendar came into use for civil purposes.
A Roman historian, Quintus Curtius Rufus, describing a ceremony in 333 BC, writes: The magi were followed by three hundred and sixty-five young men clad in purple robes, equal in number to the days of a whole year; for the Persians also divided the year into that number of days.
[6]After the conquests by Alexander the Great and his death, the Persian territories fell to one of his generals, Seleucus (312 BC), starting the Seleucid dynasty of Iran.
Based on the Greek tradition, Seleucids introduced the practice of dating by era rather than by the reign of individual kings.
Priests had no Zoroastrian historical sources, and so turned to Babylonian archives famous in the ancient world.
For example, in Achaemenid times the modern Persian month 'Day' was called Dadvah (Creator), in Parthian it was Datush and the Sassanians named it Dadv/Dai (Dadar in Pahlavi).
The tenth-century astronomer Abu'l-asan Kusyar noted that during the reign of Osrow II (AD 589–628) the sun entered Aries in Adur.
Traditionally it is said that the Caliph Omar reintroduced the Persian calendar for tax collection purposes.
The research and creation of the Khayyam calendar was financially supported by Jalal Al din Shah.
Before Khayyam's calendar, Norooz was not a fixed day and each year could fall in late winter or early spring.
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.
On March 10, 1976 (20 Esfand 1354), Shah Mohammad Reza Pahlavi introduced the "Imperial calendar" that measured the first year from 559 BC, the beginning of Cyrus the Great's reign and the foundation of the Achaemenian Empire, rather than 622 AD, the Hijra of Muhammad.