http://www.blitzbasic.co.nz/codearcs/codearcs.php?code=298
And this describes the algorithm
http://vsg.cape.com/~pbaum/date/jdalg.htm
http://vsg.cape.com/~pbaum/date/jdalg2.htm
And this might be useful
http://vsg.cape.com/~pbaum/date/date0.htm
http://vsg.cape.com/~pbaum/date/analy1.htm
And this describes the algorithm
http://vsg.cape.com/~pbaum/date/jdalg.htm
http://vsg.cape.com/~pbaum/date/jdalg2.htm
And this might be useful
http://vsg.cape.com/~pbaum/date/date0.htm
http://vsg.cape.com/~pbaum/date/analy1.htm