This plugin shows the date and time in the panel, and a calendar appears when you left-click on it.
You can also have a look at 'orage', which is a more advanced calendar.
The original plugin was written by Choe Hwanjin. Remco den Breeje wrote the 0.4.x and 0.5.0 versions. Diego Ongaro was the maintainer from 2008 to 2010.
To open the calendar, simply left-click the plugin on your panel. Left-click on it again to close the calendar.
Note: The information on which day the calendar week starts is derived from the locale.
To open the properties dialog, right-click the plugin on your panel and select Properties.
The properties dialog allows you to use a “Custom” format string for your date or time options. The format string you enter should be compatible with strftime. Here are some format string specifiers from the “date” command's help:
%% a literal % %a locale's abbreviated weekday name (e.g., Sun) %A locale's full weekday name (e.g., Sunday) %b locale's abbreviated month name (e.g., Jan) %B locale's full month name (e.g., January) %c locale's date and time (e.g., Thu Mar 3 23:05:25 2005) %C century; like %Y, except omit last two digits (e.g., 21) %d day of month (e.g, 01) %D date; same as %m/%d/%y %e day of month, space padded; same as %_d %F full date; same as %Y-%m-%d %g last two digits of year of ISO week number (see %G) %G year of ISO week number (see %V); normally useful only with %V %h same as %b %H hour (00..23) %I hour (01..12) %j day of year (001..366) %k hour ( 0..23) %l hour ( 1..12) %m month (01..12) %M minute (00..59) %n a newline %p locale's equivalent of either AM or PM; blank if not known %P like %p, but lower case %r locale's 12-hour clock time (e.g., 11:11:04 PM) %R 24-hour hour and minute; same as %H:%M %s seconds since 1970-01-01 00:00:00 UTC %S second (00..60) %t a tab %T time; same as %H:%M:%S %u day of week (1..7); 1 is Monday %U week number of year, with Sunday as first day of week (00..53) %V ISO week number, with Monday as first day of week (01..53) %w day of week (0..6); 0 is Sunday %W week number of year, with Monday as first day of week (00..53) %x locale's date representation (e.g., 12/31/99) %X locale's time representation (e.g., 23:13:48) %y last two digits of year (00..99) %Y year %z +hhmm numeric timezone (e.g., -0400) %Z alphabetic time zone abbreviation (e.g., EDT)