When assigning chapter exams, I seem to have quite often entered 12:59 PM as the due date. Clearly 12:59 is a time in the next day.
The assessment scheduling behavior is to NOT do anything except, take the minute calculation and apply it to the default end time, which is 1 hour after the current (default start time.) With no error message this can leave 1 part of the time field looking good but the hour part of the field incorrect.

Since 12:59 PM is clear the next day, this should cause a pop-up asking if the next day should be used or entered.