Sorry mate, I did a bad job of the implementation. The class code was okay, but I messed up with the double-click. This version shoud be better.

Catching input as thye type is a lot harder. Once a user goes into edit-mode our code doesn't get a look-in. We could react to the change after it is done, but they could easily exit this.

A better way maybe to lock the date cells and protect the sheet. You can then elect and double-click the cells, and you will have to unlock - add date - lock, but they won't be able to add anything else. And I would suppress the X on the calendar.