I want to write a 'Date' class that behaves like a Value Type. for example, Instead of writing a Clone method for setting properties safely, make the Date class to pass by value:

public Date Birthday
        {
            get { return this.birthday; }
            set 
            { 
               this.birthday = value.Clone(); 
            } //I want to write this.birthday = value; 
              //without changing external value when this.Birthday changes
        }

I know this is possible because System.String is a class and behaves like a value. for example:

String s1 = "Hello";
String s2 = "Hi";
s1 = s2;
s2="Hello";
Console.WriteLine(s1);  //Prints 'Hi'

First I thought writers of this class override '=' operator, but now I know that the '=' operator can not be overridden. so how they write String class?