The word "time" names one of the four dimensions in the theory of relativity. The dimensions of time together with the three special dimensions is what is called the four-dimensional manifold. For theoretical physicists the word has changed its meaning since the time of Isaac Newton,. For Newton, time and space were absolute dimensions, but for Albert Einstein, time is relative to the other three basic dimensions.
Einstein said that only time, as measurable by physicist, should interest them; but in his book A Brief History of Time (1988), the contemporary astrophysicist Stephen W. Hawking made a very questionable statement. He said that time not measurable by physicists does not exist in reality.
In saying this he contradicted himself by the title of his book. Hawkings's book is brief, but that brief period is psychological time, a period of time that is not measurable by physicists.
At this point philosophers should intervene; in the first place, because in philosophical theology, we must assume that time is everlasting -- without beginning or end -- whether any portion of this time is or is not measurable by physicists, in the second place, because time considered philosophically is the duration in which change does occur. The opposite of everlasting time is eternity, conceived as the sphere of the immutable, the timeless.
Readers will find a most interesting discussion of our psychological experience of time, in itself and in its relation to eternity, in Book XI of St. Augustine's Confessions. Eternity is, of course, not experienceable by us at all.
The psychological experience of time that Augustine discusses is certainly not the same as the time measurable by contemporary physicists, but that it has reality is beyond question.
Adler's Philosophical Dictionary (1995)
Great Ideas from the Great Books (1963)
by Mortimer J. Adler