Nowruz's timing in Iran is based on Solar Hijri algorithmic calendar, which is based on precise astronomical observations, and moreover use of sophisticated intercalation system, which makes it more accurate than its European counterpart, the Gregorian calendar.

Each 2820 year great grand cycle contains 2137 normal years of 365 days and 683 leap years of 366 days, with the average year length over the great grand cycle of 365.24219852.
This average is just 0.00000026 (2.6×10⁻⁷) of a day shorter than Newcomb's value for the mean tropical year of 365.24219878 days, but differs considerably more from the current average vernal equinox year of 365.242362 days, which means that the new year, intended to fall on the vernal equinox, would drift by half a day over the course of a cycle.
As the source explains, the 2820-year cycle is erroneous and has never been used in practice.
Nowruz's timing in Iran is based on Solar Hijri algorithmic calendar, which is based on precise astronomical observations, and moreover use of sophisticated intercalation system, which makes it more accurate than its European counterpart, the Gregorian calendar.
Each 2820 year great grand cycle contains 2137 normal years of 365 days and 683 leap years of 366 days, with the average year length over the great grand cycle of 365.24219852. This average is just 0.00000026 (2.6×10⁻⁷) of a day shorter than Newcomb's value for the mean tropical year of 365.24219878 days, but differs considerably more from the current average vernal equinox year of 365.242362 days, which means that the new year, intended to fall on the vernal equinox, would drift by half a day over the course of a cycle. As the source explains, the 2820-year cycle is erroneous and has never been used in practice.
Nowruz