Obtaining the hash value of a video, then embedding that hash value into a secure block chain is one approach that gives evidence that a specific video (or any other document) existed before some point in time.

See, for example:

https://petertodd.org/2016/opentimestamps-announcement

Editing the video after the fact changes its hash value, thereby invalidating the timestamp on the altered video.

Perhaps camera sensors could digitally sign their output using an invisable watermark? It's been an idea I've had since deep fakes became a thing.