Calculating time spend to close an incident

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I need to calculate the time being spent on an incident( excluding non working hours, waiting for customer etc). I followed this post:https://cloudcustomerconnect.oracle.com/posts/4c282086f8#148656 and I am using following expression in the column:

time_format(sum(inc_performance.rel_time), 'DDd HHh MIm')

I could not understand the result of this expression as in my report (snapshot) is attached. Only one status has value and all other incidents are having no value.

My questions are:

1. Is this approach to get the time being spent on an incident

2. how to calculate no. of hours left before SLA expires?