It seems to be the number of days since 12/30/1899? Is this a bug with the module?
No bug, that's how Excel stores dates. I've been dealing with this on a PHP-based Website, and naturally there's a Perl module for that.
http://search.cpan.org/~aburs/DateTime-Format-Excel-0.31/lib/DateTime/Format/Excel.pm
P.S.: To demonstrate that Excel actually stores the date as epoch days, open Excel and type 2014-07-11 (or any date in a format Excel recognizes as being a date) into a cell. Now format the cell as a generic number. 2014-07-11 becomes 41831.00. Date-time values get stored with the fraction of a day to the right of the decimal point.
P.P.S.: If you haven't got Excel, OpenOffice Calc behaves exactly the same way.