Can low quality video be manipulated to change the displayed time without being detectable?
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.