Formatting dates in Excel Interop format

My program generates output in Excel.

Some dates seem misinterpreted:

When you open the file, the dates are a mixture, as in the screenshot.
If I actually placed the cursor in the Excel input field (in the fingerprint) and press Enter, the cell formatting will return to the correct formatting.

I use Microsoft.Office.Interop.Excelto move data from a template Datatableto Excelthat saved in my solution.

Before putting data in cells, I change each column number accordingly using switch:

for(int i = 1; i < NumColumns + 1; i++) {
    switch(dt.Columns[i-1].DataType.ToString()) {
        case "System.Int32":
            xlWorkSheet.Columns[i].NumberFormat = "#,##0_ ;[Red]-#,##0 ";
            break;
        case "System.String":
            xlWorkSheet.Columns[i].NumberFormat = "@";
            break;
        case "System.DateTime":
            xlWorkSheet.Columns[i].NumberFormat = "[$-809]dd mmmm yyyy;@"; //"dd-mmm-yy";
            break;
        case "System.Date":
            xlWorkSheet.Columns[i].NumberFormat = "[$-809]dd mmmm yyyy;@"; //"dd-mmm-yy";
            break;
        default:
            xlWorkSheet.Columns[i].NumberFormat = "@";
            break;
    }
}

To move values ​​from datatable, I use nested loops:

//move header totals into the spreadsheet
for(int i = 1; i < NumColumns + 1; i++) {
    xlWorkSheet.Cells[1, i].value = dt.Columns[i - 1].ColumnName;
}

//move in the data
DataView dv = new DataView(dt);
dv.Sort = "Date ASC";

int rowCount = 2;
string theValue;
try {
    foreach(DataRowView dr in dv) {//(DataRow dr indt.Rows)
        for(int i = 1; i < NumColumns + 1; i++) {
            theValue = dr[i - 1].ToString();
            xlWorkSheet.Cells[rowCount, i].value = theValue;
        }
        rowCount += 1;
    }
} catch(Exception) {
    throw;
}

In the stored procedure, when I populate the Datatable, I tried to explicitly specify the type with the help of the DATETIMEfollowing:

SELECT 
"Date" = CONVERT(DATE,b.[Date])
    ...

, Excel ?

Edit

() :

int rowCount = 2;
string theValue;
DateTime dtm;
DateTime d;
try {
    foreach(DataRowView dr in dv) {//(DataRow dr indt.Rows)
        for(int i = 1; i < NumColumns + 1; i++) {
            //theValue = dr[i - 1].ToString();
            //xlWorkSheet.Cells[rowCount, i].value = theValue;
            switch(dr[i - 1].GetType().ToString()) {
                case "System.DateTime":
                    dtm = Convert.ToDateTime(dr[i - 1]);
                    xlWorkSheet.Cells[rowCount, i].value = dtm.ToOADate();
                    break;
                case "System.Date":
                    d = Convert.ToDateTime(dr[i - 1]);
                    xlWorkSheet.Cells[rowCount, i].value = d.ToOADate();
                    break;
                default:
                    theValue = dr[i - 1].ToString();
                    xlWorkSheet.Cells[rowCount, i].value = theValue;
                    break;
            }

        }
        rowCount += 1;
    }
} catch(Exception) {
    throw;
}