In pre-#Avestan Iran, the #calendar consisted of 12 months of 30 days each. Since this is obviously, clearly less than the actual length of a year, every six years, they would postpone New Year's Day by a month.

This would be like if instead of having leap years, we waited ≈120 years and declared that New Year's Day now fell on 1 February, then after another 120 years it fell on 1 March, and so on.

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Circumstantial evidence suggests that #Egypt may have used the same calendar in the distant past, but at some point the Egyptians took the eminently sensible step of adding five extra days to the end of the year.

Iran conquered Egypt in the 9th century BH (before Hejirah) and Egyptian ideas flowed into Iran. One of these ideas was the calendar, and if it was good enough for the people who built the #pyramids, it was good enough for them.

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The Egyptians were aware that the tropical year was slightly longer than 365 days, but didn't adopt a leap year system because only gods could create new days.

Iran had a long tradition of leap year adjustments, and had no problem doing so. At first, they continued as they had; every 120 years, the five extra days were pushed back by a month. At some point, they stopped doing that, and instead just repeated the last month every year; this meant each year had the same months.

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This calendar spread to #Armenia at the time a satrapy of Iran. It was used for centuries, albeit without the leap year rule (because timing things over centuries is hard). Armenian mythology has it that King Hayk introduced the calendar to comemorate his victory over the Babylonians.

But now we need to take a detour and talk about the Arab calendar.

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Al-Biruni, in describing the Indian calendar, says the Hindus measure time the same was as the Jews and the pagan Arabs. Al-Biruni did make some mistakes, but he always strove to accurately describe the beliefs of non-Muslims. As such, most historians believe that the pre-Islamic Arab calendar was lunisolar and occasionally added an extra month called Nasi'

A minority view holds that the Arabs used an Iranian-style cyclic calendar, and Nasi' referred to delaying New Year's Day.

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Anyway, #Nasi' was administered by the #Kinanah tribe, who controlled #Mecca; they had someone who job was to maintain time, called al-Qalammas.

In 10 AH, #Muhammad declared that Nasi' was forbidden, citing Qu'ran 9:36. This drastically weakened the power of the Kinanah; the one who controls time controls people, and now they no longer controlled time.

But Zoroastrian Iran would prove more resilient.

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Just three years after the fall of Mecca, Arab Muslims marched into an Iran devastated by a recent civil war. This, coupled with the fact that Islam has a lot in common with Zoroastrianism, meant that Islam gradually but steadily took hold as Iran's dominant religion. Many Zoroastrians fled to India, while some remained and are today a protected minority.

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Just as Roman culture shaped Christianity, so Iranian culture shaped Islam. It is remarked that Iran lasted millennia without needing Arabs, while after the conquest, Arabs couldn't last a day without Iranians.

That even applied to the calendars. The Arabs were using the purely #moon-based Lunar Hijri calendar, which is OK for desert nomads, but as the civil servants impressed upon #Caliph #Umar, a sedentary agricultural society needs a calendar that tracks the #sun.

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The result was that the existing Zoroastrian calendar, of 365 days, was permitted to be used for exclusively secular purposes, while holidays and religious observances would be set by the Lunar Hijri calendar. Iranian Zoroastrians eventually stopped adding extra months, but Indian Zoroastrians did it once more.

Fast forward to 457 AH, and Malik-Shah entrusts the great scholar, astronomer, and mathematician #OmarKhayyam with improving the calendar.

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Oamr Khayyam's initial proposal defined the day of the northward equinox as Nowruz, or New Year's Day. Each month lasted for as long as the sun took to traverse 1/12 of either the ecliptic or the zodiac (the information I have is unclear).

Since this required either direct observation or a whole lot of tedious and complicated calculations, it was less than ideal. Khayyam came up with a more reasonable proposal—the year still started on the equinox, but the months had fixed lengths.

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