$ perl -MDate::Simple=date -wle 'print date("2009-11-18") - 40135'
1899-12-30
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So it seems to count the days since a date close to 1900-01-01. Maybe that helps you to turn the integers back into dates.

I know that Spreadsheet::ParseExcel has the ChkType method which would tell you if a given piece of data is a date, no idea if Spreadsheet::XLSX provides similar information...