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).