Calculating elapsed duty hours

[Dan]

I'm wondering if there is a way to calculate the elapsed hours from a
beginning time to end time but only take into account duty hours not total
hours.

For example with a duty day of 8am-4pm, the start time would be 3pm and the
end time would be 8 am the next day, so the total elapsed duty hours would be
1. And if the start time was 12pm and the end time was 12pm the next day the
elapsed duty hours would be 8.

thanks
Dan