I advocate Mulla Sadra's definition of time. A Persian religious scholar of 17th century who was also a genius of the Islamic tradition of Peripatetic, Neo-platonic Philosophy. He introduced innovative theories of epistemology, ontology, theology, and human bodily and intellectual (spiritual) development.
In short, Mulla Sadra defines time as an abstraction from instability and motion inherent to the material world, with no actual, independent existence.
In other words, concept of time is a result of perception of change or motion in the world. Time has thus no actual existence. What actually exists is motion, and change/motion is an essential property of matter. Obviously concepts of time such as 'past', 'present' and 'future' don't have any actual referent in the external world. These concepts are rooted in our mind, i.e. how we mentally categorize and conceptually preserve the lasting impact of a constantly changing world on our memory. Hence, when we say a day passes, it actually means some realities no longer exist. They are "past" but the memory of that bygone reality enables us to recall it in our mind and categorize it as 'yesterday', 'past', or other adverbs or adjectives of time.
One important implication of this theory for philosophy of mind is that it points to the extra-natural essence of human mind. If human mind was not immaterial and thus constantly had its substance changed through time (like material forms), then we could have never have a concept of time, because no perception of reality would be preserved to be recalled later, giving us the sense of time. This in turn reveals that mental entities unlike material entities are in essence static and immutable.
Mulla Sadra therefore defines time as the fourth dimension of the material world. The material plane of existence is stretched in 3 dimensions of space and 1 dimension of time. Dimensions both suggest gradual, divisive and imperfect existence of the material beings as opposed to the static, inclusive and whole existence of supernatural beings that are never subject to change and thus time.
Mulla Sadra's theory of time is a part of his greater theory of Substantial Motion which is the main cornerstone of his theory of human bodily and spiritual development among others.