class Date {

private:

  long dayNumber;  // 1 = Jan 1, 1900 and so on

  // constructs a date containing the specified day number.
  // throws an invalid_argument exception is the day number supplied
  // is invalid.
  Date (long dayNumber);

public:

  // ** General Note **
  // This class ensures that dates are always valid and always lie between
  // Jan 1, 1900 and Dec 31, 2099.
  // Invalid constructor arguments, read errors, and so on result in a
  // date of Jan 1, 1900.

  // constructs a date containing Jan 1, 1900
  Date ();
  // constructs a date containing the specified date.
  // throws an invalid_argument exception if the date is invalid.
  Date (int day, int month, int year);

  // reads a date (in dd mm yyyy format) from the specfied input stream.
  // if an error of any sort (bad read, invalid date) occurs, the input
  // stream is left in the failed state.
  void read (istream &is);

  // writes a date to the specified output stream (in dd/mm/yyyy format)
  void write (ostream &os) const;

  // moves a date the specified number of days forward (positive day
  // values) or backwards (negative day values).  throws an invalid_argument
  // exception (and leaves the object unchanged) if an attempt is made to
  // move a date outside of the permitted range.
  void move (int days);

  // returns the day of the week (1 = Monday, etc.)
  int dayInWeek () const;

  // returns a date in dd, mm, yyyy form
  void getDDMMYYYY (int &day, int &month, int &year) const;

  // returns the difference between two dates.  this will be positive
  // if the other date is earlier than the date to which the method is
  // applied (and negative if the other date is earlier)
  int daysAfter (const Date &otherDate) const;

  // returns 1 if the date is greater than (later than) the other date.
  // returns -1 if the date is less than (earlier than) the other date.
  // returns 0 if the two dates are equal
  int compareTo (const Date &otherDate) const;

  // returns the date after a given date.
  // throws an invalid_argument exception if the next day is out
  // of the permitted range.
  Date nextDay () const;

  // returns the date "days" days beyond the date being opereated on.
  // "days" may be positive or negative.
  // throws an invalid argument exception if the result is out of the
  // permitted range.
  Date plusDays (int days) const;

  // returns true if the two dates are equal
  bool equals (const Date &otherDate) const;

  // logically identical to "equals"
  bool operator== (const Date &otherDate) const;

};