Determining if an xlsx cell is date formatted for Excel 2007 spreadsheets

Question

I'm working with some code that reads data from xlsx files by parsing the xml. It is all pretty straightforward, with the exception of date cell.

Dates are stored as integers and have an "s" attribute that is an index into the stylesheet, which can be used to get a date formatting string. Here are some examples from a previous stackoverflow question that is linked below:

19 = 'h:mm:ss AM/PM';

20 = 'h:mm';

21 = 'h:mm:ss';

22 = 'm/d/yy h:mm';

These are the built in date formatting strings from the ooxml standard, however it seems like excel tends to use custom formatted strings instead of the builtins. Here is an example format from an Excel 2007 spreadsheet. numFmtId greater than 164 is a custom format.

<numFmt formatCode="MM/DD/YY" numFmtId="165"/>

Determining if a cell should be formatted as a date is difficult because the only indicator I can find is the formatCode. This one is obviously a date, but cells could be formatted any number of ways. My initial attempt is to look for Ms, Ds, and Ys in the formatCode, but that seems problematic.

Has anybody had any luck with this problem? It seems like the standard excel reading libraries are lacking in xlsx support at this time. I've read through the standards and have dug through a lot of xlsx files without much luck.

The best information seems to come from this stackoverflow question:

what indicates an office open xml cell contains a date time value

Thanks!