Description
In section Function names should say what they do you're explicitly saying that you're adding a month to a date, addMonthToDate().
In section Remove duplicate code you're showing the employees list using showList().
Shouldn't that be showEmployeesList() as well, in order to explicitly tell what will be shown, so you just have to pass the type of the employee as an argument (managers or developers in that context)?
I'm not really sure about it, which is why I wanted to discuss about. Thanks for this great work!