@alicealbertinidrew No idea why the onset and offset times do not appear in the data file. However, you need to keep in mind that using onset and offset times are not reliable methods to do calculations related to the length of the video. This is because sometimes, for some reason I do not know, videos get played at a lower speed than the normal one. In these case, if you use the onset and offset times to calculate the length of the video, the length of the video would seem to be longer than it really is.
Only use onset and offset times when you are interested in clocking things that happened before or after the video. For things time-locked to the video while it is playing use the timestamps. More information about it can be found in this thread: Onset/offset times of the video stimuli are not saving - #2 by miguel_santin
Cheers!