The below video is a comparison with
Blue Iris using two different date/time displays. The top number is the BI overlay. The other, with yyyy/mm/dd, is the date/time from a Dahua camera using the camera's embedded time stamp.
As pointed out above, there is a performance hit when using the BI overlay in place of the camera embedded time stamp. On the positive side, all the cameras will display the same time when using the overlay as they are no longer dependent on camera time updates.
When using the camera embedded time stamp, camera time drift becomes important. Some cameras require an update interval every 24 hours while others may require updates every 10 minutes. Many of the older cameras allow for a 0-30 minute interval selection range with newer models up to 24 hours.
There has been some discussion on overlay vs embedded for legal purposes. With one being accepted while the other has not. This is just a point of interest as have no first hand experience to validate either.