If I understood correctly, you can first parse the text in the manufacturing date text box and then using AddYears calculate the expiration date. Something like:
System.DateTime manufacturingDate;
System.DateTime expirationDate;
if (System.DateTime.TryParse("5.9.2010", out manufacturingDate)) {
expirationDate = manufacturingDate.AddYears(2);
} else {
}
To make it more reliable you should define the format provider etc. as in
DateTime.TryParse Method (String, IFormatProvider, DateTimeStyles, DateTime%)[
^]