The fact that you can't stop excel from doing that until you are inside the document means that if you ever import anything into excel it WILL break your numbers into dates and you have to fix them manually. If you are typing a fresh doc you can obviously change the settings but I spent hours working with Microsoft support and they basically gave up saying that they used to have a beta version where you could disable it, but they never added the feature to live excel and they just gave up.
10/10 Microsoft.