I'm frequently made a little uncomfortable seeing photos or videos with people holding up a piece of paper with the date on it, as the paper is so easily forgeable for both the past and the future. Signing the date on the server seemed like an easy idea to mitigate it against the future, so I whipped this up last night. Any feedback is much appreciated.

what prevents someone to save the qr code and display it later to "backdate" an event?

Nothing- my thinking is that this would work more for (near-) real-time reporting where the pictures or video will be disseminated quickly.