ical is an old but popular calendar package written in Tcl/tk by Sanjay Ghemawat for Unix systems. ical is known for its simple, intuitive interface. It's also easy for Tcl/tk programmers to extend ical with custom functionality. It stores all calendaring information in a single flat text file per user that may be easily parsed by other programs. ical is free software released under the GNU General Public License.ical is not related to the iCalendar format standard, and does not support it.