== Required Modules and Functions ==
<b><font color='blue'>Your python script is allowed to import only the <u>os and sys</u> modules from the standard library and all the built-in functions.</font></b>
Based on the algorithm you have designed for this assignment, you should at least have the following three methods defined in your class file for Date (see later section on the purpose of each function) in order to get a passing grade for this assignment:
* tomorrow()
* yesterday()
* sum_date()
* diff_date()
You can also create additional functions to improved the re-usability of your python code by adding the following functions to earn the maximum possible mark for this assignment:
* days_in_mon()
* leap_year()
* valid_date()
* usage()
== Documentation ==