Standard solar Hijri calendar, which is the official calendar of Iran, and it was adopted in 1925 AD after replacing the traditional calendar that dates back to the eleventh century. The calendar starts from the year of Hijrah, specifically on Friday March 22, 622 Gregory, which is the beginning of the tropical solar cycle.
The Iranian calendar uses a very complex compression system, which makes it the most accurate solar calendar among solar calendars ever, and its details are as follows:
• Each great cycle in the calendar consists of 2820 years containing 1029983 days, which is the sum of 2137 simple years of 365 days and 683 leap years of 366 days, so the average length of the year is 365.24219858156 days, which is very close to the length of the tropical solar year and its average is 365.24219878177 days, and the calendar needs to Correction by deleting one day every 5 million years. To illustrate its accuracy, we compare it to the Gregorian calendar, which needs to be corrected by increasing it by one day every 3320 years.
• Each great cycle contains 21 cycles of 128 years, and one cycle contains 132 years.
• Each 128-year cycle consists of cycles of 29, 33, 33, 33 years and its total days are 46,751 days.
• Each 132-year cycle consists of cycles of 29, 33, 33 and 37 years, with a total of 48,212 days.
• Each cycle of 29 years contains 7 cycles of 4 leap years, with a total of 10592 days.
• Each cycle of 33 years contains 8 cycles of 4 leap years, and the total of its days is 12,053 days.
• Each cycle of 37 years contains 9 cycles of 4 leap years, with a total of 13514 days.