How to set Column Type when using EPPlus

You do need the DataTable column to have the right type but you also need to modify the column or cell’s Style.Numberformat.Format property.

Say you have an ExcelWorksheet named ws:

ws.Column(1).Style.Numberformat.Format  = "yyyy-mm-dd"; 
//OR "yyyy-mm-dd h:mm" if you want to include the time!