MS Excel's 1904 Date convention

[Jody Goldberg <jody gnome org>]

If we hope to share file formats we'll need to agree on common
approaches for data representation.  Picking a small detail of
Excel that we need to address seems like a reasonable starting
point.

MS Excel stores dates as numbers.  Some times as days since Jan 1
1900 (with broken leap year in 1900) and sometimes (on macs) it
stores things relative to Jan 1 1904.

This leaves us with with 2 unpretty alternatives

1) support both forms of data calculation (OpenOffice)
   and continue to export the 'use 1904' flag.
   - correct, we'll be able to produce the same values as XL
   - ugly, exposes a really silly design error in XL in perpetuity.

2) attempt to transform numbers displayed as dates into
   a different form (koffice)
   - more elegant
   - will produce incorrect results if the value is unformated

Any other options ?