Ask HN: Timestamping a DIY corona test result

Some venues accept a photo of a negative DIY corona test result as an alternative to asking to self-test at the front door in the presence of someone of the venue and waiting for ~15 minutes?

I wonder how to prevent abuse and how to make sure the test was actually recent instead. One option is to ask for a newspaper date with both the pre-test picture and the post-test picture. Is there a better method without making it a server side webcam or phonecam snap and timestamp service? The latter can also be be tampered with by feeding older/other pictures to he webcam.