Time as not-space (avākaśa)

It is very difficult to think of time, or a measure of time, without using a word related to time! E.g., "continued sequence of events" is part of the definition of time in Wikipedia. "Continued" connotes something remaining the same over time. Sequence of events is related to successive changes of something in a length of time (again!) or in space. So a definition of time may also involve space in some way or the other.

How does one know that time has passed if there is no measurable or palpable change?

One very subtle definition of time in Indian philosophy is: [that which is] not space (ākaśa). [1]

Here is a simple way to experience time as avākaśa (a-ākaśa):

Look at something blank or featureless, a white wall, for example. Go close to the wall so that it fills your entire vision.

Gently close your eyes and then open them.

If nothing has changed in space, i.e., all sensations are the same before closing the eyes and after opening the eyes, what has occurred or changed is time.

Opening and closing the eyes changes your sensation of space a little. So repeat the exercise with eyes open.

To get the same duration without using something else (like a stopwatch!), start counting mentally from 1 when closing the eyes and stop when they are fully open. Then, for the eyes-open variant, make the same count.

(Surprisingly, given the definition of time as not-space, every physical instrument that works with, or measures, time does so using change in space in one way or another. The time taken to close and re-open the eyes, a spatial activity, is called kṣaṇa [2]. Another example: the SI definition of a second [3] counts a certain number of energy changes in a Caesium atom. These energy changes occur due to subatomic transitions, essentially changes in subatomic space.)

Notes

[1] Space is the first of the five material elements in Indian philosophy. All other elements contain space.

[2] https://www.sanskrita.org/wiki/index.php?title=kSaNa [c.f. M.Bh. III.6.52-77, vana-parva, Nalopaakhyana]

[3] https://en.wikipedia.org/wiki/Time_in_physics#The_unit_of_measurement_of_time:_the_second