Why I get minus value when calculate the time between first assigned time to first reply time

Hey Qin - Based on what you've described, it sounds like you might have a few tickets that were responded to before an individual agent was first assigned to a ticket. 

For instance, in one of our ticket groups, we do not have 1:1 ownership of tickets as we're all working out of a shared queue. For that group, a ticket is typically assigned to the general ticket group rather than an individual agent within that group until we're ready to solve the ticket out. In that case, the ticket would have a negative number since the First Reply Time happened much earlier in the ticket lifecycle then the ticket being assigned to an individual agent.

However, in other ticket groups, tickets are automatically assigned to individual agents as soon as it lands in that group. For that workflow, we would expect to see a 0 or higher except in rare cases since the ticket should always be assigned to an individual agent before the end user is responded to. 

If you haven't already, I'd definitely recommend reviewing a specific ticket that is showing these negative numbers and then review the Events for that ticket as that should help explain the numbers you're seeing returned in Explore.