You may have used OpenOffice instead of Excel because some older version of Excel (2008?) was prone to handling dates wrong. That's a different problem that I've run into with Excel in the past and worked around.
Comment on Re^2: Spreadsheet::ParseXLSX returning zero
Ah, yes, for the optimist the glass is half full, for the pessimist it's half empty, and for Excel it's January 2nd (or February 1st, depending on locale)...