Converting a year from 4 digit to 2 digit and back again in C#

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.

1st solution (fastest) :

yourDateTime.Year % 100

2nd solution (more elegant in my opinion) :

yourDateTime.ToString("yy")