Physical time versus subjective time2021-03-01
Barring relativistic effects, physical time passes at a constant rate (though this is a circular statement, as "constant rate" is defined by time itself). We can use a mechanical or digital device to gauge the passage of time as a uniform change in some mechanism in the device. However, we don't experience it like this. We ubiquitously experience an acceleration of subjective time as we age. We commonly complain that a vacation "went too fast", or an unpleasent week "took forever". We have certain ideas of what a different time intervals "feel like", and when a particular instance of an interval doesn't match up with our conception, we feel a bit of dissonance. Where do these ideas come from? How often are they updated? How precise are they?
Time as change
As an initial guess, I would think the subjective experience of time has to do with quantifying change over a certain interval. Or perhaps "surprise" is a better word, because it supports the following. As children, most of the events we experience are newer to us than they are as adults. Maybe time seems to go slower in childhood because we are surprised and interested by more things. We have less concept of past. Later on, a week in which one has to deal with many unexpected and/or difficult things usually seems longer than one in which nothing "exciting" happens, a completely routine week.
Present versus past time
I think there is a distinction between the rate of time as we experience an event, and the apparent length of a past event. There is an asymmetry. For example, the 25 years of my life leading up to now don't seem so long, as here I am in the present, having lived them. But the prospect of waiting another 25 years for something seems inconceivably long - a scale I have no reference for. This is because of uncertainty. The present is much more rich than the past. The past is a fragmented, simplified concept which is "settled" in the sense that we think of it as unchangeable (I am resisting the urge to say "in the past"...) and somehow "solid".
Reality vs. desired
There may also be an element of a desired time experience. Usually, we want unpleasant experiences to pass quickly and pleasant ones to pass slowly. In fact, we want unpleasant experiences to pass immediately, and pleasant ones to last forever. So reality always seems, respectively, too slow and too fast. This does suggest an objective statement: change happens; we cannot stop or infinitely accelerate subjective time, though we are forever stuck firmly in the present moment.
Time and temperature
Similarly, we experience subjective temperature much differently than physical temperature. Though two people's internal body temperatures are usually within a degree or two, their subjective experiences can easily be searing heat versus frigid cold. This can be the case in the same ambient temperature as well. The subjective experience seems to depend on the difference between the ambient and body temperatures, preferences or mechanisms intrinsic to the individual, and one's current physical and emotional state (e.g., exercising or being angry or embarrassed cause a feeling of heat).
The concept of the past and future
But temperature is much easier to grasp and analyze than time. Time is a funny thing to conceive of, perhaps simply because it is one of the dimensions of the space through which we move. Trying to describe time is like trying to describe the experience of "forwardness" or "leftness". And yet we strongly want to analyze time, to capture, quantify and understand its passage. Why does it feel like we're moving through it? Because we have memory, and the ability to conceive of situations either in the past or future. Because we have the concepts of past and future.
The past and future are concepts. I don't know if they're real or not. When we think of the past or future, we are always bound to the present. An argument for the reality of the past is the fact that people who don't know each other will nonetheless agree on certain aspects of their concept of the past. This is History: a partially shared agreement on concepts under the heading of "the past".