in 621 prophet Mohammad start a trip fro mekka to medina city to start new islamic government and after that Moslem's based that start of new date cause they think its new age of history after persian people accept islam also persians based their starting year on prophets trip if they dont do that now Persian date should be something around year 3000
History of calendars in Persia
Throughout recorded history, Persians have been keen on the idea and importance of having a calendar. They were among the first cultures to use a solar calendar, and have long favored a solar over lunar and lunisolar approaches. The Sun was always a symbol in Iranian culture.
Old Persian inscriptions and tablets indicate that early Iranians used a 360-day calendar based on the Babylonian system and modified for their beliefs and named days. Months had two or three divisions depending on the phase of the moon. Twelve months of 30 days were named for festivals or activities of the pastoral year. A 13th month was added every six years to keep the calendar synchronized with the seasons.
The first calendars based on Zoroastrian cosmology appeared in the later Achaemenian period (650 to 330 BCE). They evolved over the centuries, but month names changed little until now.
The unified Achaemenian 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. Four days per month were dedicated to Ahura Mazda and seven were named after the six Amesha Spentas. 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. Three were dedicated to the female divinities, Daena (yazata of religion and personified conscious), Ashi (yazata of fortune) and Arshtat (justice). 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).
The calendar had a significant impact on religious observance. 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. It also clarified the pattern of festivities; for example, Mitrakanna or Mehregan was celebrated on Mithra day of Mithra month, and the Tiri festival (Tiragan) was celebrated on Tiri day of the Tiri month.
After the conquests by Alexander the Great and his death, the Persian territories fell to one of his generals, Seleucus (312 BCE), starting the Seleucid dynasty of Iran. Based on the Greek tradition, Seuclids introduced the practice of dating by era rather than by the reign of individual kings. Their era became known as that of Alexander, or later the Seleucid era. Since the new rulers were not Zoroastrians, Zoroastrian priests lost their function at the royal courts, and so resented the Seleucids. Although they began dating by eras, they established their own era of Zoroaster.
That was the first serious attempt to determine the dates associated with the prophet Zoroaster's life. Priests had no Zoroastrian historical sources, and so turned to Babylonian archives famous in the ancient world. From these they learned that a great event in Persian history took place 228 years before the era of Alexander. In fact, this was the conquest of Babylon by Cyrus the Great in 539 BCE. But the priests misinterpreted this date to be the time the "true faith" was revealed to their prophet, and since Avestan literature indicates that revelation happened when Zoroaster was 30 years old, 568 BCE was taken as his year of birth. The date entered written records as the beginning of the era of Zoroaster, and indeed, the Persian Empire. This incorrect date is still mentioned in many current encyclopedias as Zoroaster’s birth date.
Modifications by Parthians, Ardashir I, Hormizd I, Yazdgerd III
The Parthians (Arsacid dynasty) adopted the same calendar system with minor modifications, and dated their era from 248 BCE, the date they succeeded the Seleucids. Their names for the months and days are Parthian equivalents of the Avestan ones used previously, differing slightly from the Middle Persian names used by the Sassanians. For example in Achaemenian 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).
In 224 CE, Ardashir I, founder of the Sassanid dynasty, added five days at the end of the year, and named them ‘Gatha’ or ‘Gah’ days after the ancient Zoroastrian hymns of the same name. This was a modification of the 365-day calendar adopted by Julius Caesar in 46 BCE, based on the Egyptian solar calendar. Iranians had known about the Egyptian system for centuries but never used it. The new system created confusion and met resistance. Many rites were practiced over many days to make sure no holy days were missed. To this day many Zoroastrian feasts have two dates.
To simplify the situation, Ardeshir’s grandson, Hormizd I, linked the new and old holy days into continual six-day feasts. No Ruz was an exception, as the first and the sixth day of the month were celebrated separately, and the sixth became more significant as Zoroasters’ birthday. But the reform did not solve all the problems, and Yazdgerd III, the last ruler, introduced the final changes. The year 631 was chosen as the beginning of a new era, and this last imperial Persian calendar is known as the Yazdgerdi calendar.
But before the Yazdgerdi calendar was completed, Muslim Arabs overthrew the dynasty in the 7th century and established the Islamic calendar, a lunar calendar. It was outlined in the prophet Muhammad's revelation, the Qu'ran, and in his last sermon during his farewell pilgrimage to Mecca. It was the same as the old pagan Meccan calendar except that the intercalary month was eliminated, effective at the end of AH 10 (March 632 CE). Umar, the second caliph, began numbering years in AH 17 (638 CE), regarding the first year as the year of Muhammad's Hijra (emmigration) from Mecca to Medina, in September 622 CE. The first day of the year continued to be the first day of Muharram. Years of the Islamic calendar are designated AH from the Latin Anno Hegirae (in the year of the Hijra). The Islamic lunar calendar was widely used until the end of the 19th century.
The Iranian calendar was revised in the 11th century by a panel of scientists, allegedly including Omar Khayyám. The recalibration was completed during the reign of Jalaal ad-Din Malik Shah Seljuki, one of the Seljuk sultans, and named in his honor.
On February 21, 1911, the second Persian parliament tried to mandate government use of the solar years and months. The present Iranian calendar was legally adopted on March 31, 1925, the early Pahlavi dynasty. The law said that the first day of the year should be the first day of spring in "the true solar year", "as it has been" (کماکان). It also fixed the number of days in each month, which previously varied by year with the tropical zodiac. It revived the ancient Persian names, which are still used. It specified the origin on the calendar (Hegira of Muhammad from Mecca to Medina in 622 CE). It also deprecated the 12-year cycles of the Chinese-Uighur calendar which were unofficially but commonly used.
Afghanistan legally adopted this calendar in 1957 with different month names. Afghan Persian language (also known as Dari) uses the Arabic language names of the zodiac signs; these names were also used in Iran before 1925. Afghan Pashto language uses the Pashto names of the zodiac signs.
The Iranian calendar year begins at the start of Spring in the northern hemisphere: on the midnight between the two consecutive solar noons which include the instant of the Northern spring equinox, when the sun enters the northern hemisphere. If between two consecutive noons the sun's altitude rises through its equinoctial altitude, then the first noon is on the last day of one calendar year and the second noon is on the first day (Norouz) of the next year.
The calendar has 12 months with Persian names; the first six months have 31 days, the next five 30 days, and the last month has 29 days or 30 days in leap years. The reason the first six months have 31 days and the rest 30 has to do with the fact that the sun moves slightly more slowly along ecliptic in the northern spring and summer than in the northern autumn and winter. Before the modern calendar was adopted in 1925 (1304 AP), the length of the months were different each year, and a month could have 32 days. For example, in the year 1303 AP, the months had respectively 30, 31, 32, 31, 32, 30, 31, 30, 29, 30, 29, and 30 days, but in 1302 AP 30, 31, 32, 31, 31, 31, 31, 29, 30, 29, 30, and 30 days.
Solar calendar systems typically use leap years, usually every four years by adding 0.25 day to each year. This is a slight overcompensation of the Sun's the actual behavior. Instead, the Persian calendar produces a five-year leap year interval after about every seven four-year leap year intervals. It usually follows a 33-year cycle with occasional interruptions by single 29-year or 37-year subcycles. By contrast, less accurate predictive algorithms are based on confusion between the astronomers average tropical year (365.2422 days, approximated with near 128-year cycles or 2820-year great cycles) and the mean interval between spring equinoxes (365.2424 days, approximated with a near 33-year cycle).