how to auto pick up the durations in hours or days or by numbers??

Hi

I want have 2 scenario

1) I have a start date and End Time,  the duration field to auto calculate the fields to show in hrs or days based on the booked and closed date.  at the moment it is state to booked - closed , h:mm

6 Jun to 1 Jul duration, does not seem right.

2)  Have a start date only and duration in number.  how to calculate the closing date and time based on these two fields?

Thanks