When you take clips and turn them into computer playable files, whether .TS or .MOV or .MPG, .MP4, .VOB, etc., do you as the producer actually manually write into some field in a "Details" section of your video editor what the video length is, so that it can be read by the eventual user's video player? Or is this activity performed automatically by the video editor?

I'm asking because: when a mouse is passed over a file line item in Windows 7 Shell (Windows Explorer) a little box is displayed where the mouse meets the file line item and provides simple name and size properties, one of which (if it's a media file) is run length. How does that figure get there into the file (later to be read be Windows or OS X)?

Thanks for any info.