Time calculation formula

Hello,
I am trying to get formula which will help me to get the difference between two date and time. But the there is one condition that while calculation difference i need to calculate till particular time in one day and then after that calculation will start from next day.

I guess below example will clear my question properly.

Column A (Start date & time) Column B (End date & time) Column C (Available hours) Column D total time

12/13/12 5:00 12/14/12 16:00 07:00 - 18:00


So in above example i need calculate the difference in D column as per the given time i.e 07:00 - 18:00 which is in C column.

Manually i calculate this as below.

I will concider the start time from 12/13/12 07:00 hrs till 18:000. so it will be 11 hrs.

Then again from 12/14/12 07:00 hrs till 16:00 hrs bcz end time is 16:00. so it will be 9 hrs.

So my result will be 20 hrs in D column.



Is it possible to do it with help of formula??

Thanks for the quick response....really appriciated....

I have few more question here

07:00 - 18:00 is just an example. It get change per incident. But whatever the time i need to refer is mentioned in C column.

so can we make chanes in formula which will refer to time mentioned in C column instead of typing it manually everytime.

Also in many cases we need to consider the time only for Mon to Fri, some time Mon to Sat and some time Mon to Sun.

Your reply will be highly appriciated