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We take the first (or last) time specified by you, and add the calculated driving/walking/bike time and the estimated stop time (or your stop time override).
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View ETA directly in list
By activating Show more info on each waypoint under Waypoints in your settings, will make the ETA show When activated, you should see the calculated ETA directly in the waypoint list.
If you don't have that option enabled, you can view the ETA by expanding the waypoint details panel.
In the example above, the first time displayed is the time that's been manually set. That's why it has a different icon, and bold text.
The background will change when the trip extends into the next day.
Time zones
We take time zones into consideration too, and the time displayed is always the local time for the waypoint.
Time zone changes are indicated with a Tardis icon, and the number of hours (negative when you're moving back in time).
ETA on printouts
ETA will be shown on the printouts, as long as you use the default template.
Timezones
Times are always displayed in the local time of the waypoint.