You convert it when you want to display it using ToString("dd/MM/yyyy"). We don't know how you are reading the data or displaying it so it's hard to say specifically but if it is from a data reader then it'll be
string dt = ((DateTime)dataReader["Joiningdate"]).ToString("dd/MM/yyyy");
If you are binding the field in a gridview or something then you can normally specify the format via the control you're binding to.