public int compareTo(Date that) {
        if (this.year  < that.year)  return -1;
        if (this.year  > that.year)  return +1;
        if (this.month < that.month) return -1;
        if (this.month > that.month) return +1;
        if (this.day   < that.day)   return -1;
        if (this.day   > that.day)   return +1;
        return 0;
    }