The scheduled and observed times for SWR359 on 2019-02-17 are inline with GVA’s timezone being at UTC+1 and FlightXML results being stated in UTC. Were the client’s other observed time differences also approximately an hour? If the client was expecting localized times this could explain a consistent observed difference.
actualdeparturetime => 1550402090 = 2019-02-17 11:14:50 UTC = 2019-02-17 12:14:50 at GVA airport
estimatedarrivaltime => 1550403890 = 2019-02-17 11:44:50 UTC = 2019-02-17 12:44:50 at GVA airport
This estimated arrival time looks very optimist (30 minutes for a LHR > GVA).
I understand the difficulty of making accurate estimates all the time. But I need to know if I should warn my client that this situation will happen often.
In most cases, the estimated times are simply what is provided to us by either the airline, operator, or ATC, and we do not usually have any control over the accuracy/inaccuracy of those timestamps.