By definition: A persisting entity does the following two things:
1. It moves through time in the forward direction.
2. It maintains its length, width, height, mass, etc. (Note: A persisting entity maintains its physical characteristics while it moves through time.)
Objects in the real world do neither of the above two things. Real-world objects do not "persist."
Entities in the physical world are not 3-dimensional; they are 4-dimensional. (They exist within a 4-dimensional spacetime structure.) The mind perceives 3-dimensional slices of these 4-dimensional objects.
The 3-dimensional slices that make up a 4-dimensional object coexist in parallel with one another.
The mind can be described as an "entity." Although it is an "entity," it is not physical. Since the mind is not physical, it cannot exist within spacetime. (In order for an entity to exist within spacetime, the entity has to have physical dimensions. Since consciousness has no physical dimensions, consciousness cannot properly be said to exist within spacetime.)
The mind is an entity that persists.
The "present" is the mind.
The "present" is therefore an entity that persists.(1)
I indicate above that persistence is a type of motion. In my paper "Temporal Passage," I represent the mind's persistence visually as rightward motion across a timeline.
The flow of consciousness can be thought of as a sequence of mental states. Each state in the sequence necessarily persists for a duration greater than zero. (The duration of each state can, in principle, be arbitrarily short; but it cannot logically be zero.)
The Big Bang is not an "event in time." Time does not exist at the big bang singularity. In the mathematical representation of spacetime, space and time exist after the big bang singularity, but not at the singularity itself.
1. To claim that the "present" persists is simply to assert that the mind persists.