Gcal is a command line oriented program. It is usually called from the shell1 and processes given arguments that are options and commands. Options must be given before commands, i.e. you must call Gcal like this:
gcal [ [option...] [%date] [@file...] ] [command]
If Gcal is started without any options or commands, a calendar of the current month is displayed. If the calendar of a definite year is wanted, the year must be fully specified, e.g. ‘gcal 94’ displays a year calendar of the year 94, not of the year 1994.
If two arguments are given in the command part, the first argument denotes the month, and the second argument denotes the year. In case any incorrect commands are given running Gcal, the program will use internal defaults.
In the English program version, Gcal assumes the Gregorian Reformation has occurred in 1752 on the 3rd of September. See Genesis of the Gregorian Calendar, and Aspects in Internationalization, for further details.
 This is an operating system program which interprets and runs given command lines.