I'm sorry for the issues and thank you very much for your patience.
I managed to change how the system calculates dates based on hours, so now if you select 26 hours it is counted as 2 days and if select less than 24 for hours it is counted as 1 day.
Now you can select the same day on order details page.
I couldn't manage to reproduce the 0 days error but I assume it must be fixed now.
Please check the updated theme and plugin I attached to this message and let me know if there are any other issues.