Good news YEARFRAC basis=0..3 determined, 4 in progress

Good news!  I have now determined and posted the _actual_ Excel 2007 algorithms for YEARFRAC for basis values of 0, 1, 2, and 3.  For more, see:
http://www.dwheeler.com/yearfrac
This includes a minor repair in basis==1.

Excel 2007's basis==2 and 3 _are_ accurately represented by the ISO text Weir posted; basis==0 and basis==1 are NOT.

Patrick Durusau is working on basis==4, and is also trying to get official definitions of other basis systems (in particular, the ones that ISO's OOXML claims to use).

--- David A. Wheeler