It is one of the oldest calendars in the world as well as the most accurate solar calendar in use today. Since the calendar uses astronomical calculation for determining the vernal equinox, it has no intrinsic error, but this makes it an observation based calendar.[9][10][11][12] According to a proposal made by Ahmad Birashk, a complex mathematical pattern can be used to make the calendar a purely mathematical one without the need for astronomical observation. This proposed calendar has a great grand cycle of 2820 years in which 2137 years are normal years of 365 days and 683 years are leaps of 366 days, averaging a day-length of 365.24219852, over the 2820 years of the great grand cycle. This average is just 0.00000026 day shorter than the actual solar year of 365.24219878 days, making an accumulated error of just one day over 3.8 million years or approximately 0.022 of a second annually.[13]