Format string not working in C# getter

I need to format data string from format "dd.mm.yyyy" to be "yyyymmdd". I'm trying to do this following way, but this return unchanged string.

 private string _pe1;
 public string PassportEnd1 { 
    get { return String.Format("{0:yyyymmdd}", this._pe1); }
    set { this._pe1 = value; }
 }