Month is only taken into account if year is also specified. This is also how POSIX notates the format.[0]
OK? martijn@ [0] http://pubs.opengroup.org/onlinepubs/9699919799/utilities/cal.html Index: cal.1 =================================================================== RCS file: /cvs/src/usr.bin/cal/cal.1,v retrieving revision 1.31 diff -u -p -r1.31 cal.1 --- cal.1 27 Nov 2016 10:37:22 -0000 1.31 +++ cal.1 4 Sep 2018 12:05:54 -0000 @@ -42,8 +42,9 @@ .Sh SYNOPSIS .Nm cal .Op Fl jmwy -.Op Ar month -.Op Ar year +.Oo Op Ar month +.Ar year +.Oc .Sh DESCRIPTION .Nm displays a simple calendar.
