The reason this calculation is complex has to do with acceleration changes in complex paths. It is more accurate on shorter estimations and more error gets introduced on longer more complex geometries.
Do some experiments and you'll see what I mean.
Long linear travels are very easy to calculate with the distance formula at a constant velocity.
We worked on this for months and collected lots of data on performance because we didn't want to launch it calculating 12h carves as 6h.
Being off by 1min and 30 seconds seems better than not having the feature at all since it gives you some rule of thum you can optimize with.