My understanding of time begins with MAN AND TIME by Priestley. Although it's intellectually imprudent to excerpt a single sentence to summarize his survey:
"One metaphysical idea of Time: We do not discover Time but bring it with us; it is one of our contributions to the scene; our minds work that way."
Shadbolt shares similar sentiments:
Could Einstein's definition of time have been one of the greatest hindrances to the advancement of human knowledge that history has ever known?
About Time: Einstein Was Wrong
Discussions about how to define 'time' inevitably become philosophical debates. As I've noted previously, 'everybody knows what time is until they try to define it'. For the framework of this article, let's limit our discussion about time to looking at Einstein's definition of time in special relativity and contrasting that with the understanding of time in quantum mechanics.
In special relativity, Einstein defined time simply as a measure of how long an event takes, as measured by a clock. This is a sensible, straightforward measure. For example, the time it took me to read the previous paragraph, measured by a stopwatch, was 10 seconds. In special relativity, clocks are used as an objective standard for measuring the time intervals of physical processes.
The problem is that this sensible measure of time becomes time itself.
For example, if an atomic clock is observed to slow down (it registers fewer oscillations of the caesium atom at a different altitude), this is not understood as a change in the clock's operating speed. In relativity, this slowing is interpreted as a slowing in the rate of time itself.
... this mistaken interpretation commits an error that Sir Isaac Newton warned us against in his Philosophiae Naturalis Principia Mathematica.
Newton wrote, 'Relative quantities are not the quantities themselves whose names they bear, but sensible measures of them.. and by the names time, space, place, and motion their sensible measures are to be understood; and the expression will be unusual if the measured quantities themselves are meant. ..those violate the accuracy of language, who interpret these words for the measured quantities.'
Putting this concept into the modern context, even when the motion of light in a vacuum is used as the standard 'clock' to measure time, it is still just a physical entity that can be influenced by other physical processes, known or unknown. This was Einstein's biggest mistake; he employed a physical process (the motion of light) to serve as a stand-in for time itself. In Newton's words, he expressed a sensible measure of time as time itself.
French philosopher Henri Bergson, a contemporary of Einstein, disputed relativity's portrayal of time by arguing that there is a difference between time itself and what clocks display. Clocks display arbitrary fractions of periodic events such as the motion of the Sun across the sky (as shown on a sundial), grains of sand moving through an hourglass, the number of swings of a pendulum, or the number of oscillations of a caesium atom (the current standard), but this is not the physical reality of time itself. The physical reality of time is the standard against which we can compare these events.