public IAccountActivation CreateOrReplace(string username)
{
var expiry = this.currentTime() + this.activationWindow;
var activationCode = this.activationCodeGenerator();
var accountActivation = new AccountActivation(activationCode, username, expiry, false, this.currentTime);
Activations.Remove(username);
Activations.Add(username, accountActivation);
return accountActivation;
}