I entered a start time and duration but when I go to save it says "Enter a start time and duration".

This happened to me today.  It took me a few minutes to realize my mistake. We had a work holiday on Monday, but in my head I hadn't advanced to Tuesday. Therefore, I was trying to schedule a meeting in the past. The error prevented me from doing something foolish, but the description of the error wasn't clear enough for me to easily see my mistake.