Problem is ...
In Outlook, File | Options, any attempt to save by clicking the OK button (even if you have made no changes) results in the error message "the end date you entered occurs before the start date" and no changes to the Options can be made.
Solution is ...
Under Calendar, change the end of your work hours from the default 00:01 (ludicrous setting, considering that the default start time is 08:00) to, say, 18:00 - any time after the start time. All will then work as expected.
I suspect this problem is a function of what time zone your pc is operating in, but it is definitely a bug. What is more, the bug was first reported (as far as I can see) as applying to Outlook in Office 2010 as was its fix. Thus MS has known about this bug for 13 years but has so far failed to fix it (despite the fix being so very, very easy, once you know about it).
The bug is silly, and the error message downright confusing, since it occurs even if you have no appointments in your calendar, and have never entered any end date or indeed start date for anything!
So black mark, Microsoft. Please fix the bug to prevent future generations of users wasting hours of their precious lives trying to solve this problem. Do not let it persist for another 13 years.