Display a calendar
Syntax cal [-mjy] [[month] year] Options: -m Display monday as the first day of the week. -j Display julian dates (days one-based, numbered from January 1). -y Display a calendar for the current year. A single parameter specifies the 4 digit year (1 - 9999) to be displayed. Two parameters denote the Month (1 - 12) and Year (1 - 9999). If arguments are not specified, the current month is displayed. A year starts on 01 Jan.
"If everything seems under control, then you're not going fast enough" ~ Mario Andretti
Related linux commands:
dc - Desk Calculator
factor - Print prime factors
units - Convert units from one scale to another
wc - Print byte, word, and line counts
Calculate the day of the week, for any date, in your head
Equivalent Windows command: SET /A command does basic arithmetic.