Clarify date "basis" value

I've added a JIRA comment for the key topic:
 Clarify date "basis" value
 http://tools.oasis-open.org/issues/browse/OFFICE-1998

We had earlier documented what Excel did, and Microsoft has kindly submitted some additional material.  It's my understanding that this is inconsistent with the OOXML definition.  I recommend that we accept the Excel definition as correct, presume that the OOXML definition is in error, and request that someone (who has the authority to do so) submit a change to OOXML to implement the correct spec.

Sound reasonable?

--- David A. Wheeler