Use `man calendar` for details on the file format. It's more of an "on this day in history" type list than actual timeline.

The data itself (LOTR dates) first appeared in 4.4BSD (though it was part of the file `calendar.history` [ http://svnweb.freebsd.org/base/stable/2.0.5/usr.bin/calendar... ]), and has since passed to FreeBSD (where the LOTR dates were split off into a separate file [ http://svnweb.freebsd.org/base?view=revision&revision=11... ]) and subsequently Mac OS X.