<< I searched for iCal with Google and there are indeed two iCals, one is the well known (and current) Apple iCal and the other is the one referred to above which has been a bit of an orphan for a while but does have a few people working on it. >>
I can't get tcl/tk(/c) ical to compile & run anymore, so ive partially created a pure tcl that's .calendar/user.tcl compatible. Presently, it can display most of .calendar but not alter it.
