Pasting from previous post that was handled horribly circa 2013, "My company recently moved all email to Outlook Web App.  I am in the process of setting my settings, and in the Calendar, it gives you the option to set your "business day".  Unfortunately, since I work third shift, my "day" passes midnight.  When I try to set my times as 10 pm to 6 am, I get an error message saying the end time cannot be after the start time.  I know I am not the only third shift worker out there.  Is there a way to tell the Web App to let my third shift hours show?  I haven't found it yet.  Thanks :-)"

This is still an issue with OWA. Basically folks who work overnight are not able to set their business day as the start time is before the end time so attempts to save fail: The end time that you entered occurs before the start time. The start time must occur before the end time.

I posted a reply in the other thread where you commented on this.

https://social.technet.microsoft.com/Forums/en-US/48673574-5a61-4afa-9cc1-5f60d768bc79/calendar-for-third-shift?forum=exchangesvrclients

It's not just OWA, Outlook 2013 behaves the same way.  I have submitted an enhancement request and you're welcome to do so as well, but I wouldn't hold my breath for a solution in the near future.