Introduction - If you have any usage issues, please Google them yourself
Printing calendars (enter year, month, output month calendar, known January 1, 1900 is Monday).
(1) write a method to judge the leap year;
(2) write a method to determine how many days a month has.
(3) write a method to calculate the total days from January 1, 1900 to a certain month.
(4) write a method to output calendar of a certain month.
(5) write a test method.