Too much attention is wasted on what we did do and what we will do, and not enough on what we are doing. Age quod agis. "Do what you are doing." A excellent maxim. A non-philosopher will take it as such and then move on. The philosopher lingers and goes deeper.
Verbally a tautology, the admonition expresses a non-tautological truth: attend to what you are doing. I cannot fail to do what I am doing, but I can fail to attend to what I am doing. The admonition is in the same logical boat with "Be here now!" and "Live in the present!"
How could I fail to be here now? Where else would I be? And when else would I be? But that would be to miss the point. The tautological form of words expresses a non-tautological thought: Attend to the moment and be aware of your situation.
For a human being, to be is not merely to exist as a thing among things, but to be aware. The Being of a human being involves an element of material facticity — you are this indigent material thing right here — but also an element of transcendence in that, as aware, you are way beyond the miserable chunk of matter your awareness inhabits. You are way beyond it by being aware of the not-self. The not-self includes not only everything other than your body, but also your body inasmuch as your body and its parts are objects of awareness and thus not identical to you as subject of awareness. You are not merely a thing in the world, but also, as the subject of awareness, a being for whom there is a world.
As for living in the present, this is not a mere biological living. As a bit of nature's fauna, how could you biologically live other in the temporal present? To live in the present, as per the admonition, is to attend to the present, to impede the outward scatter of your thoughts, to bend back the outward intentionality (object-directedness) of mind to the present moment and its contents. You draw in your thoughts from the diaspora of the past and the future and the elsewhere in space and the elsewhere in general and bring them home. You could call it 'bringing it all back home.' You could call it spiritual intro-version, or swimming upstream to the Source of thought's river. ("Man is a stream whose source is hidden" (Ralph Waldo Emerson).
The Being of the human being is a living, but not a merely biological living, not a mere living as understood by the objectifying natural science of biology. The ineluctable subjectivity ingredient in the Being of human beings cannot be understood from the point of view of biology.
Consider now the sentence 'I am hungry' asserted by BV. It is true now at 12:45 PM. What is it about? It is about BV, a publicly identifiable person. What does it predicate of BV? It predicates the property of being hungry. The predicational tie is signified by the copula 'am.' Does this copula express merely the object BV's instantiation of the property? No, it also expresses the speaker's awareness that he himself is hungry. Property-possession in a human being is more than a merely objective relation. This fact complements the earlier one about the ineluctable subjectivity of the Being of human beings. Both the Being and the Being-propertied of human beings is unlike anything else in the world.