If you see a response time that says 00:00:00 that means that both the inbound email and the reply happened outside of business hours and hence no business hours have passed. 

We mark these as 00:00:00 response times so as not to penalize an agent who is responding outside of business hours. 

These 00:00:00 time stamps are excluded from the email response time statistics. If your business hours are 9am to 5pm and an email arrives at 8:50am and your agent responds at 9:20am on the same day we would mark that off as a 20 minute response time as only 20 minutes of business hour time has passed.