Finding overlapping time instances

Hi

I am trying to work out how to calculate the number of overlapping instances based on ranges of time.
Basically I have a number of desks that connect to jobs and then disconnect. What I am trying to do is work out the capacity load on the desks- to see how many desks are connected to different jobs at the same time.
I have tried one possible solution in row F but this just seems to return a true vale all the time.
Some desks do not connect to a job at all- and sometimes the data is missing from column b or c- I may have to filter these out.

Any suggestions are gratefully received

regards