A .NET DateTime value is just that: a date and a time, you can't have the one without the other.
However, when you display the value, you can select what you want to display:
myDataGridView.Columns[MyDateTimeColumn].DefaultCellStyle.Format = "dd/MM/yyyy";
If that isn't working for you, then I'd suggest that it's your "magic number" that's causing it - .NET use a zero based offset, and most spreadsheets have the DateTime value in the first column.
If that doesn't fix it, then use the debugger to see exactly what you get from your spreadsheet - if the column is passed as a string instead of a DateTime then no amount of formatting will change it as the DGV will display it as the original string all the time.