G is the result of a measurement. It is different than Pi, because it will not fall out of the language itself. You can derive Pi without ever picking up a circular object, but you cannot find G without measuring the interactions of objects. Like G, time is a result of measurement. That said, I am actually willing to entertain that time and cognition might be inextricably linked in the same way that green and eyesight might be inextricably linked. Of course 500nm EM wavelengths probably existed before eyes and a brain that could detect them, but the notion of green did not. Time might be similar. Time might be a transformation of a physical characteristic into one that is useful for cognition (In this case, it'd be related to entropy, I suppose). The physical reality that underpins time probably existed before cognition, but time itself as experience might be a product of our brains.