setNumFormat problem with Spreadsheet_Excel_Writer (formatting dates)

Hi, after a few tries I managed to use Spreadsheet_Excel_Writer for my purposes. Everything works fine so far except the formatting of dates. For example if I define $format_date->setNumFormat('DD.MM.YYYY'); and later I write to the spreadsheet $worksheet->write(2, 2, '31.01.2003',$format_date); then the date formatting looks correct in excel (when viewing the properties of the cell). But if I compare the dates in a given list with some date in order to show earlier or later events nothing is displayed... Only after reentering the dates _manually_ the cells seem to apply the format. Then the comparison works. Does anybody know a solution? Stefan