I had a thought. And it's probably not a novel one, but maybe something I'd want if it's already out there.

I was hiking a few days ago, and found myself asking the question: "If I start hiking now, at one point do I have to turn back so that I can be back by X." The solution is fairly simple: keep track of how long youv'e been walking, and if doubling that time puts you past your return time, you have to turn back.

So it ends up being something like:

if ( now() + ( time_elapsed * 2 ) > start_time+max_duration ) then alarm()

This, however, assumes that it will take you exactly as much time to go out as it will to come back. That's not always the case, though, being that often when hiking you'll be walking up a mountain and then back down. The return trip will probably take 75% as long, or maybe even less. So it might be that your logic is more like this:

if ( now() + ( time_elapsed + ( time_elapsed * 0.75 ) ) > start_time+max_duration ) then alarm()

Maybe this is just a feature I've never bothered to read on for every digital watch made. But still, sort of an interesting idea, particularly if you're worried about getting back to a critical social function or something.