16 is either a number from 1 to 12,
17 a lower case month name,
18 or a lower case three-letter prefix of a month name.
26 is omitted, the current month or year is used.
27 If only one argument is given, and it is a number larger than 12,
28 a calendar for all twelve months of the given year is produced;
29 otherwise a calendar for just one month is printed.
31 produced is that for England and her colonies.
38 .B /usr/local/plan9/src/cmd/cal.c
40 The year is always considered to start in January even though this
41 is historically naive.
45 refers to the early Christian era,