You have an ATC assigned airborne time of 2018 UTC (2118 local RAK) due to weather, so ba.com and the app calculate the estimated arrival time using this plus the planned flying time. Normal procedure would be to board roughly on time and then send a "Ready" message to the flow management computer which will see if there is any improvement that can be made. This is only possible once you are boarded, doors closed and ready to push immediately if required.