Considering the extent of our abilities to modify videos after a recording is made (After Effect, Blender and all those post-editing softwares out there), it's

Considering the extent of our abilities to modify videos after a recording is made (After Effect, Blender and all those post-editing softwares out there), it's hard to prove in-video other than a livestream as you mentioned. The only solution I can see would be to use an impartial third-party with legal weight to attest the current time to be as declared on video and on paper. Another idea would be to make and film a transaction on a public system (Bitcoin blockchain?) with a proof of the transaction and its timestamp?