two simple, related ideas
A human mirror is—simply put—using a human somewhat like a mirror. They can be used for looking around corners (e.g. if someone walking perpendicular to you cuts the corner close to the wall, you know it’s also safe to do so), seeing further (e.g. checking to see if the person at the top of the escalator is running to catch the train), or getting a completely different angle (e.g. when walking behind someone, judging the reaction of someone walking towards you to the person in front of you, to figure out if you should be worried).
These can theoretically be chained (for example, the person at the bottom of the escalator can see that someone at the top is not running, and I can see the person at the bottom, so I now know the train I just heard is going the other direction), but in practice aren’t reliably employed by enough people for chains to be useful. Even if more people were actively using them, they would quickly break down past about two bounces.
A person clock is just using someone else to tell the time. For example, on your way to school you pass someone you usually see much further along your route so you’re probably running a bit late. This generally assumes others are a bit more regular than you, which—for me—is certainly the case.