10
A DateTime value doesn't know about a format - it's just a date and a time. You can create a new DateTime value with the relevant information:
DateTime expiry = new DateTime(Convert.ToInt32(ddExpYear),
Convert.ToInt32(ddExpMonth),
1);
... but how that is "saved" is entirely up to you. If you give us more information, we may be able to help you more. You can format it to a string easily enough:
string formatted = expiry.ToString("yyyy/MM");
... but that may not be what you're after.