The command "cal -m" dose not work, but should. Writing just "cal" works. Seems like there is a problem with the flag parsing. I tested it also with Ubuntu bionic, the bug is present there too.
ProblemType: Bug
DistroRelease: Ubuntu 17.10
Package: bsdmainutils 9.0.12+nmu1ubuntu1
ProcVersionSignature: Ubuntu 4.13.0-16.19-generic 4.13.4
Uname: Linux 4.13.0-16-generic x86_64
NonfreeKernelModules: zfs zunicode zavl zcommon znvpair
ApportVersion: 2.20.7-0ubuntu3.1
Architecture: amd64
CurrentDesktop: GNOME
Date: Sun Nov 12 14:17:52 2017
InstallationDate: Installed on 2017-06-01 (163 days ago)
InstallationMedia: Ubuntu-GNOME 17.10 "Artful Aardvark" - Alpha amd64 (20170531)
SourcePackage: bsdmainutils
UpgradeStatus: No upgrade log present (probably fresh install)
works fine for me in focal. still an issue?