// Implement any legitimate calendar date later than 12/31/1752, or else an 
// "illegal date' of the form 0/0/0.  An attempt to use a constructor
// to implement a date that is not a legitimate calendar date will
// result in the date 0/0/0.

public interface DateInterface
{
   int getMonth();  // return date's month (0-12)
   int getDay();    // return date's day (0-12)
   int getYear();   // return date's year (>= 1753)
   
   //  return number of days from date to di2, which is negative if
   //  di2 is earlier, zero if they are the same date, and positive if di2
   //  is later.  Throws an exception if the date is 0/0/0.
   int daysUntil(DateInterface di2);  

   // Return day of week of the date as a string from
   // {"Sunday", "Monday", "Tuesday", "Wednesday", "Thursday", "Friday", 
   //   "Saturday"}.  Throws an exception if the date is 0/0/0.
   String dayOfWeek();

   // Return true if the date is a legitimate date (not 0/0/0)
   boolean isLegal();

   // Return a string that represents the date in the form m/d/y.
   // Throws an exception if the date is 0/0/0
   String toString();

   // In addition, the class should have a default constructor, which
   // produces 0/0/0.  It should have  a constructor that has int month, 
   // int day, int year parameters and constructs an object
   // with that month, day and year if they are legal, or else 0/0/0 if 
   // they aren't.  It should have a constructor that takes as a parameter
   // another DateInterface object and creates an object with the same
   // month, day, and year.
}