Proposal for basis (YEARFRAC etc.)

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

We want OpenFormula to be able to represent spreadsheets of the future and also
existing spreadsheets.  Remarkably, the OOXML spec is unable to represent
existing Excel spreadsheets with financial/date functions like YEARFRAC,
because its  definitions for "basis" are incompatible with Excel's.
The OOXML spec also fails to cover some very important basis conventions.

We can do much, much better.

I propose that basis values be assigned like this:
0..4 : Excel 2007 as they ACTUALLY ARE.  This enables easy transition.
5..31: reserved (future Excel)
32..36: OOXML spec'ed basis algorithms.  basis 34 = basis 2, basis 35 = basis 3,
  since those happen to be identical.
37..63: Reserved (future OOXML)
64..96: Reserved - another range
97+: Other algorithms as necessary.  I believe we've identified a few needed
that are quite uncovered.

This way, if someone is reading in an actual OOXML file (as opposed to a file
written by Microsoft Office 2007), they could trivially translate it by
adding/subtracting offsets.  Since no one uses OOXML at this time,
and it's not clear that they will, I think that's a reasonable approach.

I find it remarkable that OOXML is unable to represent Excel 2007 files,
either their binary or their XML save formats.

--- David A. Wheeler