Zellers Algorithm and dynamic calendar

This submission is based around Zellers Algorithm (whcih calculates what day of the week a date falls on) There are some other "zellers algorithms" on the internet that do not work, this one is from the MEI Decision Mathematics 1 book (about page 1 I think). From this code I have created a calendar that can show a months worth of days from the year 0 to the year 9999 (though they're Gregorian dates not Julian for dates, though that could be an option for the future!) Oh and it can calculate leap years as well