I figured it out.
I was running the code below and changing the formatting of every cell to "@". This is because my page is actually a generic report processor, and if the data started with a character that Excel interpreted as a function (e.g. "-", "+", etc.) then it would cause problems. The previous version of the controls however still exported a date in a date format however. After upgrading to the newest version however, dates were exported as their Excel numeric equivalent.
sender, GridInfrastructureExportingEventArgs e)
I added a check for a datetime data type before changing the format and this fixed my scenario.