Could you give an example of the sort of reply you are expecting?

In other words, what is the smallest amount of present time that can exist in order to differentiate the past and future?

It is some tens of milliseconds. At least that's the time scale on which consciousness exists, according to neuroscience. The "present" is simply what we are conscious of.

In other words, what is the smallest amount of present time that can exist in order to differentiate the past and future? — JohnLocke

In atomic engineering terms it is very small - a trillionth of a second? As to what the limits are...some say time itself is ultimately granular and discrete.

Time can be divided three ways, but only by the observer.
Past, present and future describe different present moments to different observers, depending on their relative speed and position in space/time (thanks Einstein).
Let's boil it down to present-then and present-now.

It rained on Tuesday = present-then (past).
It is raining = present-now.
It will rain on Tuesday = present-then (future).

Perhaps the subjective present moment is not 'static' as your analogy suggests but dynamic and continuous, since, according to this premise, time is a unidirectional linear collection of present moments (at least to the observer).
The subjective present therefore doesn't have a 'size' objectively, as it's experience differs for each observer.
If there exists an 'objective present', where present moment is one continuous thing rather than discrete portions of the whole, then it's size would logically be all of space and time... I think.

Now is a relative term used to describe a variable amount of time. It is undefinable by a system of measurements.

In other words, what is the smallest amount of present time that can exist in order to differentiate the past and future? — JohnLocke

Time has a physical limit in that nothing could happen in less than the Planck time - 10^-44 seconds.

But then even physically to differentiate past and future gets complicated. You would have to start factoring in the time it takes light to travel and so bring news of a difference. It takes about 8 minutes for the light (and gravity) of the sun to affect the earth. So if the sun went supernova, our present wouldn't change until 8 minutes later.

Then if you are talking about psychological time, it takes about half a second to consciously integrate a change and so update our running image of "the present".

We don't really notice that processing lag because we can respond to quite complex events in a faster habitual fashion within a fifth of a second. And what smooths out our experience of "the present" even more is that we build an anticipatory sensory expectation ahead of every coming moment. So half a second out, we are already forming a prediction of what "the present" should feel like.

For instance, we know we are about to turn our head to look towards something. So already we are subtracting away the motion to our view that we are about to cause - it feels like we are turning rather than that the world is spinning. And we have an expectation of the general scene we should discover due to our familiarity through memory.

So the psychological present moment is not some instant snapshot deal but a complex neural construction that starts by us "peering into the future" and then "working out a settled interpretation of what just happened". It spreads itself out over at least a second and then "the present" is however it washed up according to our memory.

The smallest temporal discrimination we can make is much finer grain - down to 20ths of a second for sharp onset/offset stimuli where we are focused and know what to expect. Attention can do "post-processing" to identify a particular brief signal, but at the expense of then losing sight of whatever else was going on in that half second or so "frame".

A good example of just how grainy our time perception actually is, and how much it is dependent on interpretation or expectation, is the cutaneous rabbit experiment - https://en.wikipedia.org/wiki/Cutaneous_rabbit_illusion

I suspect the present is about as big as 'here' is in size.

1) There is no future time. What we call the future time is a possiblity that we imagine in memory (memory already in the past).

2) The present is the past pressing forward. There is no way to separate them from each other since time is continuous and indivisible.

The present is a pretty big gift.
But... does it fit in Santa's sleigh?..

It's as big as 'here', so of course it fits in the sleigh. All timelord technology anyway. Anybody ever check Santa for a double heartbeat or wonder where he finds all those off-world companions he's got up there?

I opine that existential time is not a mathematical continuum. If it were, then the present would have 'measure zero.' But then how could you read this sentence? Note how the meaning of the beginning blends with an anticipation of the meaning to come. Physics time is not human or existential or lived time. For us the future dominates, wearing a dress made from the rags of the past. Through us the future carves up the present. This body is the tool through which space is carved into the shape of a desired future. And this still-being-born future has a shape influenced by the past from memory. Just think of our inherited language. We name ourselves and what ought to be in a tongue we did not choose, in a tongue that developed over generations of suffering and insight.

I've got to admit, Santa's been alive for a good two-hundred years, maybe more.
The only sufficient explanation is that he can regenerate, so he must be a time-lord.
Instead of a sonic screwdriver maybe he's got a sonic slingshot so he can deliver presents through people's chimneys??
It's all hypothetical at this point.

Specious time, that is, the minimal unit of conscious time in which an event can be said to be perceived naturally as present, is between 2 to 5 milliseconds. This is associated with the oscillatory movements of cells in the upper cortex.

Time can also be understood directly, phenomenolgically, which I suggest is more fundamental an experience of time than via pre-packaged quantitative abstractions. We have a primoidial consciousness of the passage of time. The nowness of the present is differentiated within itself. The present is not properly understood as an isolated ‘now’ point; it involves not just the current event but also the prior context framing the new entity. We don’t hear sequences of notes in a piece of music as isolated tones but recognize them as elements of an unfolding context. As William James wrote:”...earlier and later are present to each other in an experience that feels either only on condition of feeling both together”

nice!

I red your post -past,I write my post-present, you will read my post-future. But all this is in my mind not in yours or in other people's minds.This proves that there are not any time periods for people as a whole.

I red your post -past,I write my post-present, you will read my post-future. But all this is in my mind not in yours or in other people's minds.This proves that there are not any time periods for people as a whole. — vesko

From observation one can say that we experience changes in memory that feels like personal (psychological) time (duration). This is the time we experience. Since it is all memory, we can call it what we will but most c specifically it is there sense of memory evolving.

Other have mentioned this, but part of the difficulty of pinning down the present is that we never just "exist" in the present. Experience irreducibly involves temporality. Phenomenologically, we experience not only the "present" but also a retention of the past and a protention of the future that anchor us to the world. It is not incorrect to say that we live in the present only by living a little in the past and future as well.

Phenomenologically, we experience not only the "present" but also a retention of the past and a protention of the future that anchor us to the world. — darthbarracuda

Indeed. Well said. And this is naked for whomever just looks.

Answer to this question depends simply on the question that we make about the "past", "the future" and "the present".

I opine that existential time is not a mathematical continuum. If it were, then the present would have 'measure zero.' But then how could you read this sentence? — ff0

Back to Zeno. At any particular instant the arrow is at rest. How then can it move?

I agree with you that the mathematical continuum is not an accurate model of the nature of the real world. What's interesting is that physicists are trained to think of an instant of time as a real number t. What's the acceleration at time t, what's the temperature?

The math helps them to craft interesting theories. But the world is definitely not the same as the mathematical real line. The world is not a set of dimensionless points of space and instants of time.

Time can be infinite. and so the past is infinite and the present is infinite and the future is infinite. It is only our conscience that experiences time linearly. Studies into how time and space behave have shown that in the right conditions such as a black hole, time and space can be warped and changed and so if you are in a scenario where you have fallen into a black hole you may experience a few minutes where back home on earth, we have experienced 50 years in the exact same time. and so the question of how small the present is can be answers with a few more questions. How long does the human conscience experience the present? Does time run linearly or is that just how we experience time? is time truly infinite? and if so, that will answer your question. it's infinite.

Buddhists have a doctrine of non-self. The idea being that there is no experience of a permanent unchanging thing that we could call an essence or self. For them the self is a convenient fiction that enables us to live together. It is a fiction that covers over their ultimate reality. I'm not too sure about this but I would imagine that their view of time is that the present is a convenient fiction also. In other words, perhaps the present is an ultimately meaningless social construct that is only meaningful at a superficial level due to its pragmatic usefulness? I'm not sure what it would be like to experience the present...

I agree with you that the mathematical continuum is not an accurate model of the nature of the real world. What's interesting is that physicists are trained to think of an instant of time as a real number t. What's the acceleration at time t, what's the temperature?

The math helps them to craft interesting theories. But the world is definitely not the same as the mathematical real line. The world is not a set of dimensionless points of space and instants of time. — fishfry

Right. The theoretical picture of nature reminds me of a grid that's loosely projected on an everyday experience of just being in the world with furniture and other people. I agree that this grid is not the thing itself. I'd also stress that even the objectively real is something that has to be filtered out of tangled personal experience. We basically scrub everything personal away. In fact we have mortal being with particular faces and sense organs and histories, etc., experiencing things. But (for good reason) blend and filter such experience into an image of the object, the public. This kind of thing probably goes even deeper, to the use of words like 'experience' and language in general. We pretend/assume that 'experience' has a fixed, universal meaning, etc. As others have said, we project being on becoming. But that's no final statement either. 'Fail again. Fail better....Till nohow on.'

It depends on the context.

How would one answer this question?

I guess the present or any division of time for that matter has to be meaningful. Read this way then, as other posters have said, the present is the time length necessary for us to perceive changes in our world.

Another way of looking at it would be the smallest length of time that can be measured. @apokrisis says that the smallest unit of time is the planck time (about 10^-44 seconds). I'm guessing here but I think that's the smallest time period of the fastest repetitive phenomena in the universe. We can't measure time smaller than 10^-44 seconds or perhaps no physical change can occur faster than the planck time.

A previous person suggested that measuring the present is like measuring 'the here'. Is this not genius?

It depends on your views on time really.

Traditionally we understand it to be an instant, or duration-less (as someone like Augustine would say, it's like a knife edge separating the past and future). However, some people would venture to say that the present has some extension to it and that what currently exists is more than an instant but still very short. If you take the idea to the extreme, you have the block universe, where all moments exist. Take it back a notch and you have the growing block universe, which is smaller but it would still be pretty big.

There is also the specious present, the present that figures in our perception, which is a little bit bigger than an instant but that all depends upon our brain processes. Relativity also talks about simultaneity, which refers to the way we order events in spacetime, where we could call the "now" we live on the set of events in which are simultaneous to us. However, I imagine that that is not what you are looking for.

Personally, I don't think that there is such a thing as a present "moment" or "time" at all, so questions about its duration are meaningless. You don't need times or moments in order for things to change.

the present is a time of no dimension. The past is negative, the future positive (or vice verse) whilst the present of the zero on the timeline.