How do we sense the flow of time?

Space (Akasa Pannattis) and time (Kala Pannattis) are a conceptual or perception based construct (Pannattis). Since the notion of time is a perpetual construct this is not what is called an Ultimate Truth. The passage of time is perceived by an observer thought his experience of the "world". We perceive a difference on when one experience happened vs another.

See: Time and space: The Abhidhamma perspective, Professor K. N. Jayatilleke Memorial Lecture 2003 by Y. Karunadasa.. Also The Dhamma Theory by Y. Karunadasa which discusses Pannattis.

So, how is it possible that we have a sense of the flow of time? How do we know that there was a past?

According to my experience (not according to doctrine), I have a sense of time in the following ways:

• Sometimes (i.e. at some present moments) I am imperfectly remembering something from the past (a feeling, a person, a place); i.e. I'm presently conscious of a memory or imprint
• Sometimes (i.e. at some present moments) I am planning (trying to foresee) a future action or experience, e.g. "I will plan to change my regular routine in order to go to a lesson next Thursday evening because I want to repeat a previous/learned experience i.e. that I successfully went to a previous lesson on the previous Thursday"
• I label time (a clock for the time of day, a calendar for the day of week) and therefore notice that the 'current time' changes; for example I'm writing this in the day-time, but as I was writing it the thought occurred to me "although it is visibly day-time now (I see day-light) there is such a thing as night-time, when it's dark and when in the past I was outside and saw the nighttime sky with stars and moon"
• Sometimes I see things which are in motion (e.g. a person walking or a plant blowing in the wind); my sense of sight is trained to perceive motion; and my mind has been trained to associate motion with time (a change in location over time)

The motion of objects (something which was in one location and is now in another location), and counting period cycles (e.g. planet's orbit, the motion of a pendulum, the ability to count ocean waves on the shore, or to count breaths, or to count foot-steps) is I think more-or-less the definition of 'time'.

The time flowing is a purely a mental projection.But how?

Imagine yourself in a field,

here

you do not have a watch

it is dark sun is not coming up

can you guess the time now?

This is an actual experiment to explain how humans experience time.Time for humans is very easy to understand,

watch a movie for 2 hours and sit down,start meditating for 2 hours.You will feel like it is 4-5 or 5 1/2 hours.

The experiencing of the flow of time is relative to what you are experiencing now.If what you are doing is exiting time is faster to your mind (Like a date with the love of your life). But if what you are doing is boring time is slower to your mind (Like school)

So the conclusion is the feeling of time passing by is a complete illusion of mind thinking.If the mind was asleep like you do in the night you will not feel it at all.

(If there was no clock in your house can you say how many hours you slept last night?)

So what about the Past?

Past is not a real thing.Your brain can't even fully memorize a single event.What it does for this amazing magic to happen is very simple. It is more like how you studied back in the school.

Your brain/mind remember the most important events of a memory and fill in the blanks to make the rest of the memory.But you can't find a difference between the memory and the real event.Because what your mind makes up is all you can remember.

For example try to remember the last time you went to see your grandmother.

Can you remember what shoes you were wearing?

Can you remember who you met before going there that day?

I guess you will not be able to.Brain did not recorded those things in the permanent memory.Because none of those small things were important to make the memory in the first place.

Past is a conceptual construct that only seems solid in the absence of careful examination. But if you look closely you will see that Past is assembled by the mind from multiple cues (as is Present by the way).

The experience of second-to-second flow of time is a byproduct of chitta-vrtti, the associative cycle, when each subsequent dharma (~thought) comes by association with the previous one. The sense of relatedness of the two dharmas is what creates the illusion of flow.

Disclaimer: this answer represents my own experience and understanding and is not meant to reflect an official position or to be consistent with views of any Buddhist school.