what is time?

Albert Einstein
:   Zeit ist das, was man an der Uhr abliest.
[Time is what a clock measures.]
Isaac Newton
:   Nam Tempus, Spatium, Locum & Motum, ut omnibus notiſſima, non definio.
[I do not define time, space, place, and motion, as being well known to all.]
(480–411 BCE)
:   τὸ πολυτελέστατον ἀνάλωμα τὸν χρόνον.
[The most precious thing a man can spend is time.]
(384–322 BCE)
:   ὥσπερ οὖν εἰ μὴ ἦν ἕτερον τὸ νῦν ἀλλὰ ταὐτὸ καὶ ἕν, οὐκ ἂν ἦν χρόνος.
[As, if the now had remained the same, time would not have existed.]
Augustinus Hipponensis
[Augustine of Hippo]
:   quid est ergo tempus? si nemo ex me quaerat, scio; si quaerenti explicare velim, nescio.
[What, then, is time? If no one ask of me, I know; if I wish to explain to him who asks, I know not.]
Marcus Aurelius
:   Ποταμός τίς ἐστι τῶν γινομένων καὶ ῥεῦμα βίαιον ὁ αἰών· ἅμα τε γὰρ ὤφθη ἕκαστον, καὶ παρενήνεκται καὶ ἄλλο παραφέρεται, τὸ δὲ ἐνεχθήσεται.
[Time is like a river made up of the events which happen, and a violent stream; for as soon as a thing has been seen, it is carried away, and another comes in its place, and this will be carried away too.]
Julian Barbour
:   We shall come to see that time does not exist.
Henri Bergson
:   Le temps est ce qui empêche que tout soit donné tout d'un coup.
[Time keeps everything from happening all at once.]
Hector Berlioz
:   Le temps est un grand maître, dit-on; le malheur est qu'il soit un maître inhumain qui tue ses élèves.
[Time is a great teacher, but unfortunately it kills all its pupils.]
Carol Burnett
:   Comedy is tragedy plus time.
Charles  Chesnutt
:   Time touches all things with destroying hand.
Benjamin Disraeli
:   Time is precious, but truth is more precious than time.
Benjamin Franklin
:   You may delay, but time will not.
Benjamin Franklin
:   Remember that time is money.
Merrick Furst
:   The biggest difference between time and space is that you can't reuse time.
Ben Hecht
:   Time is a circus, always packing up and moving away.
(460–370 BCE)
:   Χρόνος ἐστὶν ἐν ᾧ καιρός, καὶ καιρὸς ἐν ᾧ χρόνος οὐ πολύς‧
[Time is that wherein there is opportunity, and opportunity is that wherein there is no great time.]
Aldous Huxley
:   Time, as we know it, is a very recent invention. The modern time-sense is hardly older than the United States. It is a by-product of industrialism — a sort of psychological analogue of synthetic perfumes and aniline dyes.
Franklin P. Jones
:   Time is a versatile performer. It flies, marches on, heals all wounds, runs out, and will tell.
Stanisław Lec
:   Życie zabiera ludziom zbyt wiele czasu.
[People find life entirely too time consuming .]
Abraham Lincoln
:   The best thing about the future is that it comes only one day at a time.
Groucho Marx
:   Time flies like an arrow. Fruit flies like a banana.
David B. Norris
:   How you spend your time is more important than how you spend your money. Money mistakes can be corrected, but time is gone forever.
(43–18 BCE)
:   Tempus edax rerum.
[Time is the devourer of all things.]
Theodore Roethke
:   Time marks us while we are marking time.
William Shakespeare
:   But thought's the slave of life, and life time's fool; and time, that takes survey of all the world, must have a stop.
William Shakespeare
:   I wasted time, and now doth time waste me
Bob Talbert
:   Time neither subtracts nor divides, but adds at such a pace it seems like multiplication.
Henry David Thoreau
:   As if we could kill time without injuring eternity!
H.G. Wells
:   Clearly… any real body must have extension in four directions: it must have length, breadth, thickness, and duration…. There are really four dimensions, three which we call the three planes of space, and a fourth, time. There is, however, a tendency to draw an unreal distinction between the former three dimensions and the latter, because it happens that our consciousness moves intermittently in one direction along the latter from the beginning to the end of our lives.
Steven Wright
:   Everywhere is in walking distance if you have the time.
unknown: :   寸金难买寸光阴
[An inch of gold cannot buy an inch of time.]
unknown: :   The speed of time is one second per second.
unknown: :   Time heals all wounds — except deadly ones.
unknown: :   Wasting time is an important part of living.
unknown: :   Time is an illusion perpetrated by the manufacturers of space.
unknown: :   What does "it" mean in the sentence, "What time is it?"
Tom Stoppard
:   Eternity is a terrible thought. I mean, where's it going to end?

Lots of fancy words for what could be stated more simply, or can it? Simple concepts are often the most difficult to explain. Many times, there's one notion that we carry around with us and another, more technical and specific, that's used in physics. What time is it? Pretty simple question, no? . Now tell me again what time is. Is my definition any good? Well, of course it is. Otherwise I wouldn't have written it down. Time in the physical sense is always relative and never absolute. That is to say, we always consider time as the interval between two events and not as some cumulative measure of this abstract thing we talk about when when ask, "What time is it?" When did time begin? In the year 1 CE? Hardly. That's just a cultural fixed point. Do you think Socrates walked around thinking, "I can't believe it's 535 BCE already. I can't stop writing 536 BCE on my checks"? When the aliens land on Mount Shasta they'll have a different chronometer reading in their spaceships, using different units and a different zero point. Furthermore, why is it in London and in Seattle? This idea of local time is not usually what we mean by time in physics.

Have you ever heard people say "Daylight Saving Time is ending so we'll gain an hour Saturday night"? Well let me tell you right now that nobody ever gained an hour when Daylight Saving Time ended, not literally anyway. If you were in the hospital, nearing death at 1:59 AM on the last Saturday of October in the United States would you have lived an extra hour if you died two minutes later at 1:01 AM? Would you say you went back in time? No way. There's time — in the sense of how long it takes for something to happen — and then there's time -- in the sense of "What time is it?". Time in the former sense, as used in physics, is measured with a stopwatch or an interval timer, while time in the latter sense is determined with a wristwatch or clock. In the course of your study of physics you'll have to understand the difference between the two meanings of this one word.

Time is a measure of the interval between two events. Time is also a cultural construct whereby an event can be associated with a series of numbers. When the quantity called time appears in a physical equation, it is always referring to the measure of the interval between two events.