I have an Excel file with dates in them. The internal numeric value can be retrieved correctly, but DateUtil.isCellDateFormatted does not recognize it as a date format. Debugging, it fails at public static boolean isADateFormat(int formatIndex, String formatString) { where formatString is "reserved-0x1c" I could find no good documentation on that format string (and unfortunately cannot share the file in question), but it seems to be a date format used by Chinese versions of Excel. LibreOffice correctly shows it as a date (but OS X Preview does not -- shows up as a number).
Are you able to give us a few examples of the formats it produces?
The original file (with formatString "reserved-0x1c") is a customer file and I cannot share that. I tried editing it down to cut out everything except the date cells, but it seems that this changed how the dates are being represented. I used Microsoft Excel 2003 on Windows XP (I do not know which version of Windows or Office the original was made with). I am attaching this edited file anyway, because it does not parse with POI either. It still displays as a date in LibreOffice (07/23/2013) and now also in OS X Preview (as AD2013年7月23日, whereas in the original file it displayed as 41478).
Created attachment 30614 [details] stripped down version of the problematic file, edited with Excel 2003 on XP. Now shows a different behaviour (but still not working properly with POI).