Created Unassigned: DateTime value appears as numeric value in Excel [13648]

When I try to set cell value from DateTime class and then create Excel i see it there as numeric value, where i need it as date value.

So when i do something like:
```
cell.SetCellValue(DateTime.Now);
cell.CellStyle = datetimeCellStyle;
```

the way i'm creating the format is:
```
var dt = document.CreateDataFormat();

var cellStyle = document.CreateCellStyle();

cellStyle.DataFormat = dt.GetFormat("MMM-dd-yyyy");

return cellStyle;
```

I'm using NPOI 2.1.3 with Excel 2013

Is there any workaround for this issue to have date cell when exported to excel?

Thanks.