Groupng time in pivot tables showing difference between start and end date

[newbieexcelgirl]

I have start query dates and end query dates and need to find the resolution time in between. This is for around 2000 incidents and could be more every month.

I have added a column to take away the start date from query date but the format of the time, even when i go in custom and do mm/dd/hh is not coming up correctly.

What I would like to do is create a pivot table and show the time in groups (right click group in pivot table) and show all queries in groups of 10 mins, 20 mins etc.
I then need to create a chart based on the averages.

Could you advise how i could do this?

I really really appreciate your help!!