Just wanted to add that I remember discussions on leap year handling (esp. 29th Feb 2000) together with a bug in MSOffice calc. There it was wrong and correct in calc, the result was that it was changed in OOo calc (at SUN times AFAIR) to be compatible with the behaviour of the 'de-facto standard'. I just hope this is not the task we discuss about, there is no good solution :-( Just my 2cents...
-- ALG
