[acqq]

The drifts come from different speed of videos compared to the speed of the videos for which the titles were done.

E.g. if one subtitle is made for 24 frames per second speed (classic film speed) and you have a video presented in 25 frames per second (common in Europe). The original two hours video in 24 fps is then 5 minutes shorter in the Europe-origin version. Or the opposite: the subtitles for 120 minutes would at the end appear 5 minutes before!

Apparently there are some other speed changes, for which I don't know how they happen.

I have done one such correction once, using the linear function to model the correction ^based on the target times of the first and of the last title.