"Delivered" simply only means these 4 things and only these 4 things:
1 you sent an iMessage to one of the registered identities associated with that person ( mobile number or email)
2 apple's iMessage server well received that message
3 apple's server relayed that message on
4 at least one of the recipient's registered devices were connected to the net, in a state able to receive iMessages, and message has been recorded by apple as having been delivered to a device.
It is my experience that "delivery" in the above context has not proven to have ever failed.
Time of delivery is set by the time the receiving device received the message.
Received in this context means downloaded to any registered device.
Nothing in the above implies the recipient has touched the message to "open" it and show a "message read" status (if indeed "read" status is set to even be shown).
iMessage preview length can be setup and length of viewable message changed. In which case you can view at least some of the iMessage on the home screen if you set it up that way. You can therefore view messages without any overt clicking on them, in that case, not sending back to the sender a "message read" status.
If "read" status is set to be shown, then the time the message was actually physically clicked on to read it becomes the time "Read".