calendar internally relies on datetime and uses datetime.date. datetime.date can't represent date more than 9999.12.31 so some logic is broken for it(normally the week list should be of length 7, date outside the month will be (0, x) pair but this is not the case for 9999.12). This won't affect prmonth since it doesn't do center formatting. We can change to that behaviour to fix this. calendar_pryear_9999 tires to fix this.