Long ago, the place where I worked used “a Julian Calendar”.The Julian Year was 1000 days long, with the last number of the Gregorian Calendar Year.
Example: ‘Jan 01, 1961 would be 0-0-1-1.
Next day, 0-0-2-1 all the way to 9-9-9-1, then that next day begins the “2” year, 0-0-1-2.
As to what happens when you’ve reached “9-9-9-9”, I do not recall that....
Note, an easy historical eye of 1000 days, was the length President Kennedy’s time in office.
Huh? No, not 1000 days. 365.25 days.
https://en.wikipedia.org/wiki/Julian_year_(astronomy)
https://en.wikipedia.org/wiki/Julian_calendar
https://en.wikipedia.org/wiki/Gregorian_calendar#Gregorian_reform