The classic way to prove a photograph is taken after a certain date is to use a recent newspaper, but what would be best for the opposite? I thought about a structure/building that's been demolished after the picture was taken, but it seems hard to go through with. Any ideas?
I'd imagine it depends on how you define "simplest" and whether or not you know you need the proof when you're taking the picture, or if you're just attempting to date a photo that already exists.
For example, if I found a picture of someone standing next to Richard Nixon, I'd know the picture was taken before April 22, 1994.
On the other hand, if I wanted to take a picture of myself right now and prove that I'm taking this picture before January 8, 2013, you could do that to a large degree of accuracy by taking the photo in front of a public time-and-date-telling landmark, like the New York Stock Exchange or something. On the other hand, getting to New York may be kind of hard, even though the idea is in principle conceptually simple.
The cogency of photographic content for dating has never been high and has plunged in the digital era. No longer is a newspaper a slam-dunk. We cannot prove in a mathematical sense nor can a photograph ever be a single trusted source for a decision where the stakes are high enough e.g. a cause for war. At best a photograph will only be a part of a body of evidence to warrant a high level of certainty.
To take wmjbyatt's suggestion, a photograph that includes Nixon - it could be with a close look-a-like or at a wax-work-museum or against a blow-up poster or entirely fabricated digitally.
The best we can achieve are justifications that sufficiently rationalise the belief of the date of the photograph to an individual fit for a given context. The standard of the justification for a murder trial is clearly higher than that required to verify your friend ate a 2 ft. long hot-dog.
The simplest routes for personal belief:
If taking a photo to convince another then your simplest route would be to seek an independent 3rd party to be the source of the photograph with no motivation to manipulate the content. For example:
The more interesting question is who are you planning to kidnap, and what will my cut be?!