Reading Date from Excel File using OpenXML

I'm making a library that uses OpenXML in C# to read excel files. I can read a cell text and numbers just fine, but when it comes to dates there's a problem. There's the type "date" for the cells, but apparently Excel 2007 doesn't save the dates in that type, so I can't tell if the value I'm reading is a date or not, instead it appears to use styles.

How could I detect if it is a date and return the string representation of it 开发者_JS百科(ex: 29-12-2010)?

Excel stores dates as a float value... the integer part being the number of days since 1/1/1900 (or 1/1/1904 depending on which calendar is being used), the fractional part being the proportion of a day (ie the time part)... made slightly more awkward by the fact that 1900 is considered a leap year.

The only thing that differentiates a data from a number is the number format mask. If you can read the format mask, you can use that to identify the value as a date rather than a number... then calculate the date value/formatting from the base date.