Time and its Effects

Actually, Einstein didn't receive a Nobel prize for either Special or General Relativity. His Nobel prize (1921) was for an explanation of the photoelectric effect.

Apparently, Relativity was a little too far out for the Nobel Prize committee.

As to duration, it can be measured accurately by physical processes such as the signal from an Atomic Clock. Subjective measurement of duration from "sensed changes" is problematic. An hour on the beach seems shorter than 5 minutes in the dentist's chair.

...."Einstein determined that time is relative—in other words, the rate at which time passes depends on your frame of reference."...

..."In math, time can be defined as an ongoing and continuous sequence of events that occur in succession, from past through the present, and to the future. Time is used to quantify, measure, or compare the duration of events or the intervals between them, and even, sequence events."...

One is the definition of time, and the other is the nature of time...

There are two conditions where time passes more slowly due to relativity, and this has been experimentally verified. Time moves more slowly in a gravitational field and more slowly for an object in motion.

A muon is a particle that is created when cosmic rays strike the upper atmosphere. It decays in 2.2 microseconds. Even travelling at the speed of light, it could only travel about 2200 feet, yet the muons reach the ground. This is possible because the muon's time is passing much slower.

https://en.wikipedia.org/wiki/Muon

Time in a gravitational field passes more slowly. A correction for this effect is programmed into GPS satellites.

https://www.astronomy.ohio-state.edu/pogge.1/Ast162/Unit5/gps.html

Whatever its nature, meaning or experience, there is never enough of it.

IMHO:

Time is impossible to measure without motion.It is subjective,or as A.E. said,relative.

The measurement of time is a human construct that is irrelevant,except that it simply puts a name to the concept of time.

Spacetime is simply an energy field that is in constant motion in all directions.

A mass that is in the path of this energy field creates a drag on the speed ,and it takes longer for time to pass,which generates what we refer to as gravity.(Similar to magnetic lines being bent by a conductor generates electron movement within the conductor)

The more massive the object,the harder it is for the spacetime field to pass through it and the more acute the bend.

If the speed of the mass increases, it increases in one direction only, bending more field lines,creating even more drag on the spacetime energy field which is moving in all directions simultaneously,thus becoming more "massive."