Calendar Wiki

A cyclic era date notation counts the completed leap cycles since the calendar's epoch and the completed years within the current leap cycle separately.

For the Gregorian calendar (and most reform proposals based thereon), the cycle length is 400 years exactly, although its Computus for the date of Easter repeats over a much longer interval. The year 2020 CE) or MMXX AD by traditional reckoning, i.e. 12020 HE, 6020 AL, 1398 AHS, 109 (Minguo, Juche), Reiwa 2, 2563 (Thai), would then be noted as something like 6.020 (one-based, ordinal counting) or +5-020 (zero-based, cardinal counting), because we have completed five full leap cycles since the epoch, even if the modern leap rule has only been introduced in 1582, 4.382 or +3-382. The years 1999 through 2001 were 5.399, 5.400 and 6.001 or +4-399, +5-000 and +5-001 accordingly.

The Julian calendar has a leap cycle of just four years, which is too short to be useful, but its weekdays only repeat after 28 years, which is at the lower bounds of a useful cyclic era: 2020 would be something like 73.04 or +72-04.

The Dee or Dee-Cecil calendar has a leap cycle of 33 years. This means that a very efficient and practical way of counting eras would group 3 cycles into an era of 99 years, thereby using 2 digits in the date notation perfectly: 2020 would then be +20-44 instead of +61-07, for instance.

Followers and analysts of the Maya calendar use a convention like this for its short and long counts, one of which infamously overstepped in 2012.