When following a track, when the estimated time of arrival goes past midnight, I think that the ETA shows how many days ahead the time is, eg +1 in front of the time.
But it does not take the time zone into account correctly.
For example if the time zone is 10h ahead of UTC, the time is currently 09:00 and the estimated time to end is 2 hours, the ETA shows +1 11:00.
The ETA should just show 11:00.