Talk of time flowing is perhaps strange. It is not that parts of timelines themselves are moving along the lines; it is objects within them. But if time otherwise has no intrinsic "content," what is it that we call "time" that is "flowing"?
To avoid the picture of parts of time moving, or at least postpone this mystery, let's focus on, "Time flows," as metaphorically (if nothing more) predicating an action or event of time. (By "action" I do not mean a deed, but a positive determination from time, not something that "passively happens" to time.) Actually, I'd be hard put to explain a temporal event with no content: I would look for a change, but then I'd have to confront the idea of time changing, or of being different (even for some "static" 4-dimensionalist kind of reason) at different subtimes.
So as far as temporal action goes, I'd say that, "Time flows," means more that time, of itself, moves the objects in it, forward down time's lines. We flow through time, and not only on account of specific physical dynamics that we experience and engage with, but due to the whole ambient "purpose" of time overall.
This picture of time is, I hope, arbitrarily consistent with any well-evidenced scientific theory of empirical events. In an inherently dynamical universe, for instance, space too, and then rather the spacetime manifold as a more integrated system of functions, comes equipped with a force of action on its contents. On the other hand, that spacetime acts on its contents might seem less likely on a relational (Leibnizian) model, for such a model has spacetime "only" as relations between other objects and events. However, this relation-type itself could be construed as "acting on" its relata, perhaps. (Quick attempt: assume with Frege that concepts are Concepts, that existence is second-order predication (predication on other predication), so that the Concept of time can cause itself to be instantiated as much as it is instantiated by its physical relata; maybe then we have room to speak of time acting on its relata instead of just passively relating them.)
At any rate, if spatiotemporal relations are sui generis conceptually, we seem to have a sort of quasi-substantivalism on offer: at least in the mind of God, the irreducible relation-types of spacetime are compressed into mental objects, alongside all other monadic or polyadic structures of the relevant form.
Addendum. Kant seems to have had a humdrum thought (not necessarily false, though) about "time flowing":
Space and time are quanta continua, because no part of them can be given, without enclosing it within boundaries (points and moments), consequently, this given part is itself a space or a time. Space, therefore, consists only of spaces, and time of times. Points and moments are only boundaries, that is, the mere places or positions of their limitation. But places always presuppose intuitions which are to limit or determine them; and we cannot conceive either space or time composed of constituent parts which are given before space or time. Such quantities may also be called flowing, because synthesis (of the productive imagination) in the production of these quantities is a progression in time, the continuity of which we are accustomed to indicate by the expression flowing.
He also said:
For change does not affect time itself, but only the phenomena in time (just as coexistence cannot be regarded as a modus of time itself, seeing that in time no parts are coexistent, but all successive). If we were to attribute succession to time itself, we should be obliged to cogitate another time, in which this succession would be possible.