What is the time?
Aristotle speculated that time could have an effect of speed but he also said that motion can be slow or fast but not time! Aristotle had no means of knowing Einstein's theory of relativity, according to which change in the speed of time is possible. Similarly, when Einstein was working on the development of the theory of general relativity and he made a revolutionary proposal that the curvature of space is caused by the effect of mass.
But Einstein did not know at that time that the universe was expanding. The expansion of the universe was discovered by Edwin Hubble 13 years after Einstein published his theory of "simple relativity". If Einstein had knowledge of the expanding universe, he would have included it in his theory of general relativity.
Conceptually, it is easier to show the curvature of space in the form of a slowly expanding field due to the effect of gravity in the expanding universe. One of the most dramatic aspects of our universe is that it is expanding and the presence of motion, force, and curvature time-spans in the expanding space.