What's confusing about the OP's post was that he says the arrival time was off by half an hour. Does he mean that the initially posted arrival time was wrong or that the Garmin was wrong even at the end of the trip? I'm going to assume that he means that the originally posted arrival time was way off.
I have seen this happen with our Garmins (one of which is a 760LMT) when the original starting point isn't on a road, such as in a mall parking lot or in an RV park. Occasionally the GPS will act as if it's computing an "as the crow flies" time to the destination which will be way off from any real time. But this situation is quickly corrected as soon as the device finds itself on a road it recognizes and the arrival time is updated.
Other than that we, also find the arrival time to be very accurate and it is continually updated as we drive. However, as was already noted, if you don't update the device's assumed driving speed data the initial arrival time may still be a bit off. If you are driving on highways with speeds >75 mph the difference between your real arrival time and the initially assumed one can be large if you don't change the speeds at which it assumes you are driving.
