you can reset and make default codes and settings on your nokia 5800xm as fallows:
take all your important backup ( contacts, music, application softwares).
2. Switch off your phone.
3. Press and hold three keys together ( Camera key. Call key Green and Cancel key red) and switch on your phone and keep pressing the three keys untill you see the logo of Nokia hand shake.
Now you phone is resetted and change time Zone all other setting need be happy.