This gives a total of 683 leap years every 2820 years, which corresponds to an average year length of 365 683/2820 = 365.24220 days. This is a better approximation to the tropical year than the 365.2425 days of the Gregorian calendar.
The current 2820 year period started in the year AP 475 (AD 1096).
This “mathematical” calendar currently coincides closely with the purely astronomical calendar. In the years between AP 1244 and 1531 (AD 1865 and 2152) a discrepancy of one day is seen twice, namely in AP 1404 and 1437 (starting at vernal equinox of AD 2025 and 2058). However, outside this period, discrepancies are more frequent.