Approve user

Hello,
After a user has been registered I am sending the following:
http://MyDoman.Com/Account/Approve/3620e45b-d899-4990-8ea5-74cbf2af8153?Key=C3EA110078422FF19478A8B45A7C34F4
Basically, the guid after Activate/ is the user ProviderUserKey. The key is a Hashed MD5 string created from:
String.Concat(user.UserName, user.Email, user.CreationDate.ToString())
Then I approve the user as follows:

string test = CryptographyHelper.Hash(String.Concat(user.UserName, user.Email, user.CreationDate.ToString()));
if (key == test) {
user.IsApproved = true;
}
But shouldn't I make this URL t active the account to have a duration? For example, after 2 wee ...