Is there a way to subtract string input?

I have to input time of login and log out, then subtract the 2. to get total amount used.
just testing i tried this but gave me the error can't do it, so was thinking there might beanother way.
any help would be appreciated greatly

-dale

(example: book gives ann.easyio, so replace with your keyboard input and would be good)

  import ann.easyio.*;

   class Main extends Object
  {

     static Keyboard myKeyboard = new Keyboard();

      public static void main(String args[])
     {
        String     time_start,
                              time_end;
        int         time_work;                   
        System.out.println("Please enter the login time: ");
        time_start = myKeyboard.readWord();
        System.out.println("Please enter the logout time: ");
        time_end = myKeyboard.readWord();
        time_work = time_end - time_start;         
     }//////////////
  }//END OF class