I created two commits. One without and one with prototype example. I would choose for the example with prototype, but leave it up to you (if you accept this pr).

Would the second commit contradict the Don't write to global functions?

Good point! Maybe the first commit is the winner then. Or the native Date object should be replaced with an "own object"?

Agreed with @vsemozhetbyt. Let's not write to global functions :) I like your first commit though, can you remove that commit from this branch?

Deleted commit with prototype :)

👍 for this change.

• Functions should start by a verb to differenciate infinitive and simple past
  example: var dateSet = setDate()

• Function names should indicate the argument order
  example: function addMonthToDate(month, date)

Thanks that's a great improvement!