withMonth method
Make a new date object with changed month
Original date object remains unchanged
This method is NOT safe
Throws DateException on problems
Note: For changing at once use copy() methods
Note: You can chain methods
Implementation
@override
Gregorian withMonth(int month) {
if (month == this.month) {
return this;
} else {
return Gregorian(year, month, day);
}
}