try to concatenate your DateTimeString using "//" or Better:
DateTime DOR = new DateTime(day, month, DateOfRetirement.Year);
(Dim DOR as DateTime = new ... or something like this in vb.net)
And Afterwards save the ToString() value in a String, not a DateTime!
String s = DOR.ToString();
(Dim s as String = DOR.ToString or something like this in vb.net)
at the end assign the string to the table cell.