Looking at the source [0], the date field is specified as the last date of the Julian calendar used by that locale. Since LN is the Julian calendar and will never change, it's presumably set to the max date value that the calendar.h date type can represent (AD 10,000) minus the day-counting delta between the two calendars over that period.
I look forward to being dead long before the Y10k problem crops up.
I tried to find an explanation and only found a fellow seeker of the truth [0] that also couldn't find the answer [1]. He probably tried to search for it more than me.
I found the Apple source code for ncal [2], but no answers there. Also it is funny that Latin line has the month entered in octal (05).
I look forward to being dead long before the Y10k problem crops up.
[0] https://opensource.apple.com/source/misc_cmds/misc_cmds-32/n...