What the delay figure means
The number is the punctuality the vehicle reports against its own timetable. How to read it, including when it turns out to be negative.
Where the number comes from
The delay you see on a vehicle or a departure is not something OVatlas works out. It is the punctuality the vehicle reports about itself: the difference, in seconds, between when it is somewhere and when its own timetable said it should be there. That value travels in the same messages as the position, and OVatlas shows it to you, rounded to whole minutes.
Two things follow. First, the figure belongs to one trip and one timetable: the comparison is against that vehicle's own schedule, not an average for the line, the day or the operator. Second, it is a snapshot. It changes the moment the vehicle reports again, and over the course of a trip it can move either way — recovering at a quiet stop, growing in the rush hour or at an open bridge.
A negative value means early
A minus sign means the vehicle is running ahead of its timetable. That sounds like good news, and for a train gaining a little slack it is, but for buses and trams running early is a problem in its own right. Turn up at the scheduled time and the trip may already have gone. Operators therefore work actively to prevent early departures, and you can see it on the map: a vehicle that is ahead will often sit at a stop until its time comes round.
A small number in either direction does not mean much, either. Traffic lights, boarding, and one busy stop will shift a trip by half a minute without anyone experiencing it as a delay. A delay becomes interesting once it grows, and keeps growing across successive reports — that is the pattern that should make you look again at a tight interchange.
Why it differs from the operator's own app
Put OVatlas next to the operator's app and the numbers sometimes disagree. Neither is broken. The feed carries punctuality in seconds; OVatlas rounds that to whole minutes and shows a small deviation as simply on time. The operator's app applies its own thresholds, and it reads the vehicle at a different moment besides. The same bus can therefore produce two different figures without either side doing anything wrong.
On top of that, an app may be showing you a prediction — an expected departure time at your stop, projected forward over the rest of the trip — where OVatlas shows what was reported. On a trip running to time that difference is invisible. On a trip whose delay is growing it is not, because a projection runs ahead of the last report. Both pictures are correct; they answer different questions.
What the number is not
The delay is not a forecast. It says how the vehicle is doing now, not how it will be doing four stops from here. A bus two minutes down may have made that up by the next stop, and a train running to time may still come to a stand ten minutes later. Treat the figure as a thermometer rather than a promise, and for a tight interchange look at the marker itself: where is the vehicle, and is it moving.
Nor does the number say anything about cancellations. A trip that is not running reports no delay at all. For what is going on with a line or a stretch of route, the disruption reports are the right place to look; OVatlas shows those separately in the disruptions panel, with the operators' own messages and the planned engineering works.