Docs can be found at http://incrtcl.sourceforge.net/iwidgets/iwidgets/datefield.html and http://purl.org/tcl/home/man/iwidgets3.0/datefield.n.html
A functionally equivalent but tcl-only datefield widget is available at tcl-only datefield. This tcl/tk only version is now a part of tklib.
Category Command, a part of incr Widgets