Clocks are a great analogy. As pointed out, the problem is in verification. Say everyone has a clock and they're all showing different time. How do you actually get to the ground truth? Observe and measure planetary motion? That's too much hassle. This is where the analogy cracks a bit (or does it?). What you want is for your clock to show the same time as others' clocks do, so you join a group where you synchronize your watches. Other people join groups too, possibly with different synchronizations. Fact has become convention, subordinate to localized social utility.