This is a very fundamental question. He never defined the notion of duration. So there are aspects of time that haven't been fully studied, in my opinion. One of the great questions in physics is whether there's some sort of invisible framework in which everything unfolds. Newton introduced the notions of absolute space and absolute time.
Newtonian time is like some invisible river that flows uniformly for ever.
The trouble with this is that we can't see this invisible framework, all we see are things moving relative to each other. This is the relational viewpoint, as opposed to the absolute viewpoint of Newton. The challenge has been to create a theory containing genuine relationships between genuine things, and not relationships between real things and unobservable things. That's what I've spent a lot of my time working on. It's given me the ideas which I'm trying now to develop into a complete cosmology, a complete explanation of what the universe is.