My credit card processor requires I send a two-digit year from the credit card expiration date. Here is how I am currently processing:
If you're creating a DateTime object using the expiration dates (month/year), you can use ToString() on your DateTime variable like so:
DateTime expirationDate = new DateTime(2008, 1, 31); // random date string lastTwoDigitsOfYear = expirationDate.ToString("yy");
Edit: Be careful with your dates though if you use the DateTime object during validation. If somebody selects 05/2008 as their card's expiration date, it expires at the end of May, not on the first.