cal - print calendar cal [ month ] [ year ] Cal prints a calendar. Month is either a number from 1 to 12, a lower case month name, or a lower case three-letter prefix of a month name. Year can be between 1 and 9999. If either month or year is omitted, the current month or year is used. If only one argument is given, and it is a number larger than 12, a calendar for all twelve months of the given year is produced; otherwise a calendar for just one month is printed. The calendar produced is that for England and her colonies.

Try .EX cal sep 1752

/sys/src/cmd/cal.c The year is always considered to start in January even though this is historically naive.

Beware that .L "cal 90" refers to the early Christian era, not the 20th century.