Create the possibility when designing the calendar overall of a month that does not have a set number of days. I don't mean a leap year, which is still a set number of days just on a more complicated schedule, but a truly variable month. Some years it is 20 days, some years 30 days, etc. Ideally this would be done without requiring that you specify the number of days in each year (so that a 3,000 year history does not need 3,000 entries), but would allow you to customize the number of days in a specific year that was important.