The time listed on the read receipt is the time zone that your organization is set to. If you see a read receipt, the first thing you should do is match the 'created' time on the read receipt. You can match this to the specific comment time by looking for the comment creation date in your ZD instance. If your instance is in a different time zone you will have to mentally adjust the time zone on the ticket to verify a read receipt which usually means just making sure the hours and minutes match.
Each time the Notify requester of comment update is triggered, a pixel will be attached to the comment. The first time that an email is read by an end-user that does not use the ZD instance as agent the read receipt will be created. Hence, 3 comments will result in 3 read receipts if all 3 comments are opened by the recipient. We block the IP addresses of agents who use ZD to lower the risk of a false positive.
Once our API is notified that the recipient has opened the email, we use ZD’s API to post the read receipt as a private comment update. The time it takes us from receiving the pixel and calling ZD’s API takes milliseconds. On the other hand, ZD’s may throttle back API requests depending on overall network load. In addition to that when ZD has a service outage we save all pixels and once ZD’s service is restored we post all read receipts.