Do all of these media files: .mov, .mpg, .mp4, .ts, and .vob, have a tag that indicates play length? If not all, which ones? And can they be read by any machine?
QuickTime Movie File (.mov)
We see the duration in:
Movie Picture Experts Group - MPEG-1 (.mpg)
— Source: Code Project - MPEG Audio Info
Movie Picture Experts Group - MPEG-4 (.mp4)
This is a container for different file formats, you need to figure out the format first.
Once known, you can then either read or use a formula to get more information, it's either:
— Source: Wikipedia - MPEG-4
Movie Picture Experts Group - Transport Stream (.ts)
Boils down to the same thing...
It's meant for transmission of already existing video.
— Source: Wikipedia - MPEG Transport Stream
Movie Picture Experts Group - Video Object (.vob)
Again boils down to the same thing...
— Source: Wikipedia - VOB