What indicates an Office Open XML Cell contains a Date/Time value?

Question

I\'m reading an .xlsx file using the Office Open XML SDK and am confused about reading Date/Time values. One of my spreadsheets has this markup (generated by Excel 2010)

Answer 1

It was unclear to me how to reliably determine whether a cell has date/time value. After spending some time experimenting I had come up with the code (see post) that would look for both built-in and custom date/time formats.