YEARFRAC information

["David A. Wheeler" <dwheeler@dwheeler.com>]

I've published detailed information on YEARFRAC, including
a detailed definition that matches what Excel _actually_ does
instead of what OOXML says (they are incompatible):
http://www.dwheeler.com/yearfrac/excel-ooxml-yearfrac.pdf
http://www.dwheeler.com/yearfrac/excel-ooxml-yearfrac.odt

We now need to decide what to do with OpenFormula; this affects
26 functions.  The 'obvious' answer is to use the definitions that
are actually compatible with Excel, since there are far more Excel
.docx spreadsheets than OOXML spreadsheets
(to my knowledge, there are no OOXML spreadsheet documents).
If OOXML decides to stay with their incompatible basis
definitions, we could assign new basis values for
those algorithms, so we could handle both OOXML and .docx.

We could assign basis values 16..31 as meaning "the same as
OOXML basis 0..15"; I think we can appeal to the definition of an
external specification if it's a standard :-).  I suggest these
be optional.

--- David A. Wheeler