How to calculate average time of answering chat and transfer rate

Content

is there any way to calculate average time of answering chat, i mean date difference between when agents/queue gets toast invitations for chat and when agent accepted the chat. i used chat_events table to locate time of agent Engagement Invitation and when Agent accepted but not able to get difference these two chat event.

i am using below expression but this doesn't work :

date_diff(if(chat_events.event = 11, chat_events.created) , if(chat_events.event = 8, chat_events.created))

also i need to check how many call transfered to another queue or agent, i can make individual report by using chat_event report but i need everything is in one report, can anyone has any idea how i can do that.