How to make correct date format when writing data to Excel

This worked for me:

sheet.Cells[currentRow, ++currentColumn] = "'" + theDate.ToString("MM/dd/yy");

Note the tick mark added before the date.

Did you try formatting the entire column as a date column? Something like this:

Range rg = (Excel.Range)worksheetobject.Cells[1,1];
rg.EntireColumn.NumberFormat = "MM/DD/YYYY";

The other thing you could try would be putting a single tick before the string expression before loading the text into the Excel cell (not sure if that matters or not, but it works when typing text directly into a cell).

Try using

DateTime.ToOADate()

And putting that as a double in the cell. There could be issues with Excel on Mac Systems (it uses a different datetime-->double conversion), but it should work well for most cases.

Hope this helps.