Using milliseconds for speed calculation, needed for Suunto Ambit gpx datafiles

I had speed spikes when importing Suunto Ambit gpx files. I figured out that these files have milliseconds in the timestamp and in QDateTime this is already available, so the changes above work fine for my files.
Sample:
2014-01-16T18:48:44.340Z
and the following timestamp could be:
2014-01-16T18:48:46.870Z