If you are exporting a date from C# to Excel using 2007, you probably will use the following:
This exports the date as a OLE Automation date recognized by Excel 2007. How ever, in Excel 2010 this was causing issues, every time I exported to Excel 2010, it said that it has to repair my document and it never displayed the values correctly as dates.
Because I was using Open XML to generate the markup, Excel 2010 has been support for Open XML. I got around the problem of using:
This exports the date as a XML Date format.
Hope this helps anyone who has the same problem!