What is time?

Ever since Einstein, we're used to referring to the space-time continuum. It's often said that Relativity spatialized time. 

On the face of it, space seems physical in a way that time does not. Take an enclosed space like a basketball. Not to mention that finite space has measurable dimensions. Likewise, we move through space. So space seems to be very concrete. 

Mind you, pure geometry deals with abstract spaces. Likewise, dreams and video games have simulated space. 

But we don't move through time the way we move through space. In one respect, time appears to be physical. Physical states and objects undergo change. A temporal process. So it seems like time is a mode of the universe. 

Likewise, it's often said that God created time when he made the universe. He made the universe with time rather than in time. There was no preexistent time. 

However, human mental states are temporally successive. But if thoughts aren't physical, then the temporality of the mental process can't be physical, either. 

And even at the level of physics, there are theories of a cyclic or oscillating universe, where there's a series of cosmic births and deaths and rebirths. Even if those theories are false, it doesn't seem incoherent in principle to say that one universe would be earlier while another universe would be later. 

If so, then time can't just be a mode of the universe, since on that view it ceases to exist when the current universe ceases to exist, then starts anew when a new universe comes into being. But to speak of a temporal series of universes assumes an overarching timeline or relative chronology that transcends any particular universe, and carries through the changes. That's what makes the model sequential. 

However, it's possible to think of time as abstract. Take a novel or script. That has a plot. One thing happens after another. But unless it's enacted, the plot remains static. 

Perhaps, then, we should say a mental process exemplifies time, a physical process exemplifies time. On that view, time isn't essentially physical, but physical states and processes represent time or temporal relations (precedence, simultaneity, succession).