Package: ncal Version: 12.1.7+nmu3 -J doesn't seem to have any effect when used together with -b:
$ ncal -b 2 1900 February 1900 Mo Tu We Th Fr Sa Su 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 $ ncal -bJ 2 1900 February 1900 Mo Tu We Th Fr Sa Su 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 But February 1900 has 29 days in the Julian calendar: $ ncal -J 2 1900 February 1900 Mo 7 14 21 28 Tu 1 8 15 22 29 We 2 9 16 23 Th 3 10 17 24 Fr 4 11 18 25 Sa 5 12 19 26 Su 6 13 20 27 -- System Information: Architecture: amd64 Versions of packages ncal depends on: ii libc6 2.33-7 ii libtinfo6 6.3-2 -- Jakub Wilk