There is a function called DATEVALUE() in excel. It is a worksheet-function, so you can enter =DATEVALUE("01.01.2010") to turn this string into the corresponding excel date value. You can apply this function to an excel worksheet after the export from the report.
In may be necessary to strip away the daynames first.
Patching the excel-export part of report.php might force the report to a certain format.
Custom certificate (plugin)
Certificate Date Format
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