system
Given the following post, respond as if you're commenting on Reddit:
human
eli5: Who or what decided the moment from which we start measuring our time?

Since this is ELI5 I’m gonna explain my question the same way. 

Imagine there is a time-hating wizard who magically removes all time-keeping instruments like atomic clocks, phones and watches, so there is no way of knowing the time right now. If then we were asked to ‘bring back’ the old time that we used (e.g. so that we start again measuring 12:00 in London exactly when it was 12:00 in London before the wizard) could this be done? Is there something physical constant (like kg or meter) that determines what time of day it is? Or, did someone just say “we start measuring time from now, and I say that it is exactly 14:32…”? 

Also, if there is this constant that allows us to know exactly when a certain time is, doesn’t that mean we don’t need atomic clocks and can just compare ourselves to this constant? Idk what this would be but perhaps when the sun is absolutely highest in the sky somewhere?
gpt
We'd use the sun like we used to. Other commenters have pointed out that noon is the easiest point of the day to measure. That's because the sun reaching it's highest point in the sky is not subtle and easy to measure. 

You wouldn't even need anything complicated. Jab a stick in the ground, and you can trace the shadow throughout the day. Noon is when it's the shortest and points straight towards the nearest pole. if you're not in the tropics. From there, you mark the arc into hours. Now you have a reference to build your mechanical clock around. 

That's how clocks began in the first place. They were mechanical sundials with the hour hand simulating the shadow of a sundial. In the Northern Hemisphere where they were invented, a sundial's shadow starts in the west, swings north, and then to the east. The same direction we now call clockwise (and used to call sunwise).