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...
