Whether the spreadsheet is using 1904 dates or not is controlled by a XML property. Its value can be the string "false" that is not mapped to the boolean "false" but to the boolean "true"... Therefore Spout was previously using the wrong date system when this property was set.
This commit adds support for dates using the 1904 calendar (starting 1904-01-01 00:00:00).
It also fixes some issues with the dates in 1900 calendar (which now correctly start at 1899-12-30 00:00:00).
Finally, it is now possible to have negative timestamps, representing dates before the base date (and up to 0000-01-01 00:00:00), as per the SpreadsheetML specs. Note that some versions of Excel don't support negative dates...
