~OLCommand :~RS calendar
~OLUsage   :~RS calendar [<month> [<year>]]

This will display a calendar, highlighting (if you have colours turned on)
the current date.  If no options are used then it will display the
calendar for the current month and year.  If the <month> option is used
then it will display the calendar for that month and the current year.
Else it will display the calendar for which ever month and year you
supply.  <month> is a number between 1 and 12, <year> is a number between
1 and 99, or 1800 and 3000.