A friend of mine is being accused for theft and the only evidence for that is a video camera of a nearby shop where he just passed normally. The time on the video matches. He says it's a set up. The court needs evidence that a video with such low quality can be changed (the time in the corner of the video). Is this something that is easy to change and are there any reliable sources that talk about this?
I'll assume the time code is at the top or bottom of the video, and off to one side. In simple video-editing software (like Adobe Premiere or even the free Apple software iMovie), an editor could zoom in on the shot to crop out the timecode.
At this point, though, the editor would have to create a fake timecode and lay it over the top of the video convincingly. Once that was done, the editor would save the video as a new file which would appear nearly the same as the original but with a new timecode.
The easiest way would be to simply add a timecode to video that doesn't have one. If you had a clean video, it's super trivial to add something to it. It would be quite a bit harder to alter something if there was already timecode put there (unless cropping would work like BrettFromLA suggests), but it could also be argued that the clock was also simply wrong on the camera.
Your best bet is to have a video forensics guy look it over for signs of manipulation and have them act as an expert witness.