The instants differ from one another, however slightly, and this is how our impression of time arises. It is as if there is a cine film and we are looking at successive frames. If we imagine such a film laid out in a line it is clear that all frames coexist. There is no time, only position. The frames at different positions effectively have different times.
Time only arises when our attention switches from frame to frame.
A clock can be considered to be an object whose position changes uniformly from instant to instant and time is then simply a measure we invent proportional to the apparent change in position. In this sense position is fundamental but time does not exist independently and is an invention of our minds. If we had no memory from instant to instant we would not form the concept of motion, or of time. The only instant of which we are actually aware is the present instant. Although we like to think of past and future instants we are never directly aware of them, only of the present, though our awareness appears to extend over a few instants.