Do clocks really measure time? Each time I listen to this quote from The man from Earth, I'm absolutely intrigued:

Dan: Time... you can't see it, you can't hear it, you can't weigh it, you can't... measure it in a laboratory. It is a subjective sense of... becoming, what we... are, instead of what we were a nanosecond ago, becoming what we will be in another nanosecond. The whole piece of time's a landscape existing, we form behind us and we move, we move through it... slice by slice.
Linda Murphy: Clocks measure time.
Dan: No, they measure themselves, the objective referee of a clock is another clock.
Edith: All very interesting, but what has it got to do with John?
Dan: He, he might be man who... lives... outside of time as we know it. 

I don't understand how can a clock not measure time? Given that all clocks agree among themselves as to their respective measurements, but isn't 1 second as much a finite quantity of time as, say, 1 litre of milk?
What makes 1 second any different than 1 litre or 1 gram as regards measurements? If it is subjective, then how come we all agree as to our respective time-spans (a day equals 24 hours for both me and you)?
Please explain what exactly IS time? Do clocks really measure or don't measure time?
 A: If you stick to the Newtonian model of universe, then — yes, clocks do measure time. But to "measure" time, you must first define the least unit of change to act as a reference model. We call this a "second". There are different ways to measure a second, but anyway, in Newtonian models of universe, a second always stays a second.
If you delve into Einsteinian (Relativistic) model of the universe, then with some lenience the quote can be regarded as true. The clocks DO measure time in Einsteinian model of the universe too, but its surprisingly faulty. Here a second becomes a person's (or clock's) subjective feeling of how much time "appears" to have passed. The twin paradox explains it clearly. Take two clocks of the finest quality. Test them 100 times and make sure they both measure time accurately. Now keep one clock with you on Earth and put the other clock on a spaceship traveling at 1/2 the speed of light. After a year, let the spaceship return to earth. Now if you compare the times on both clock's, your clock (staying on Earth) would show one year has passed while the one you put on the spaceship, would show that only 8 months have passed.
If you like sanity, stay with the Newtonian model. If you like truth, adopt insanity with Einsteinian model.
A: In a sense to be discussed below, clocks do indeed measure time, and this is a very definite experimental result that gives us an experimental definition of time.
We experimentally observe that the ratio of the rates of the same two physical processes taking place in an inertial laboratory is always the same. A clock pendulum swings a set number of times, as a rubidium atom in an atomic clock oscillates a set (generally mich higher) number of times before a certain, always the same, extent of reaction between the same chemical reagents is reached, and this number of swings and the number of rubidium oscillations before that extent of chemical reaction is always the same if the pendulum, rubidium and the reagents are at rest relative to one another. It's part of the experimentally observed predicability of the World: set up the same two experiments with the same set of conditions and the physics will be repeatable: the ration between the rates of progression of the experiments will be the same as long as the two experiments are at rest relative to one another. It is this consistency between rates of processes that lets you pull an egg from boiling water when the sands of your egg timer have run out, and to know it will be cooked a consistent amount defined by your egg timer, even though there is no direct causal link whatsoever between the timer and the egg.
Given this basic consistency, the notion of a "good clock" becomes well defined. It is simply an instrument whose behavior is repeatable enough that its rate of working relative to the physical processes around it is always the same. Note that this notion would not be well defined if relative rates between the same physical processes weren't consistent and changed randomly. Misner, Thorne and Wheeler have a wonderful discussion of the notion of "good clock" in the first chapter of their book "Gravitation", as does Ben Crowell in the early part of his book "General Relativity".
We choose a "standard" cyclic process, measure its rate (or period), and then define the "duration" of all other processes and the "time elapsed" between pairs of events as the number of "standard cycles" that complete throughout the process or between the events concerned.
Likewise, when the same ratio is computed for the same pair of physical processes happenning in different inertial frames, the ratio of their rates changes from the value it had when they were relatively at rest, and this change of ratio is given by the effect on the time co-ordinates of each frame by the Lorentz transformation between the frames. This too is a strongly confirmed experimental result, even though we guessed the right transformation grounded on symmetry and other theoretical arguments some decades before it was confirmed by measurements.
It's all yet another manifestation of the experimental result that Eugene Wigner called the "Unreasonable Effectiveness of Mathematics in the Natural Sciences - processes can be foretold and the World isn't total chaos. There is a repeatability in physics.
A: Clocks measure themselves. If you were to move forward or backward in time,the clock will not update itself,like a thermometer or GPS will.
A: I just watched "The Man From The Earth" and that quote didn't sit right with me.  The more I think about it, the more I'm convinced clocks DO indeed measure time... or at least as good as any other method for measuring it.  And the claim that a clock's accuracy can only be measured against another clock simply is not true.  I once read somewhere that we humans measure time through motion... the time it takes for an event to occur or for something to move from one place to another.  That's true for the most part.  Getting back to the accuracy of a clock, it depends on the clock but they do measure time.
     A conventional timken movement clock driven by a spring measures according to the time it takes a balanced weight to reciprocate against a hairspring.  Pendulum clocks use the swing of a pendulum.  Of course there are countless variables that can affect the accuracy of these mechanisms, but what about clocks that do not use moving parts?  Digital clocks use the oscillation of a quartz crystal.  Then there are the most accurate of all... atomic clocks, which utilize the decay of a radioisotope.  Any of these clocks can be pitted against the other, but clocks aren't the only way to measure time.
Humans can and do use both gravity and sound to accurately measure time as well.  The speed of sound is for all practical purposes constant.  If a rifle fires and someone several miles away hears it, a formula is usually used to measure how far away the rifle was from whoever heard it.  But the same formula can be used to mark the passage of time if the distance between the rifle and whoever heard it is already known.  Dropped objects can also be used to measure time.  A cannonball dropped from a 10 story building always falls at a given speed.  This example is usually used to measure the speed at which something falls, but if you already know the speed, you can use it to measure (or verify) the time it takes.  If you know it should take exactly 5 seconds and you start a clock at the point of release and stop at the point of impact and the clock reads 5 seconds, you've just verified the accuracy of your clock without using another one.  
There are many other examples... a water cistern draining through a pipe of a certain size at a certain pressure, light flowing through a fiber optic cable, radar bouncing off a target, etc.  Usually a clock is used to verify that these scenarios are correct.  However they could just as easily be used to verify that the clock is doing its job accurately as well.  
A: Clocks measure time. It's true that you normally judge a clock by comparing it to another clock, but that means nothing significant. That's obviously the best way to judge it! Good clocks all agree with each other because they are all measuring the same thing: time.
(More precisely, they measure their local time. Good clocks will disagree if they have different gravitational redshift etc. By the way, I take "clock" in the broad sense, to include rotation of the earth or any other process with known time dependence that could be used as a clock.)
Similarly we normally judge rulers by comparing them to other rulers, we normally judge mass by comparing it to other masses, etc. It's a sensible and practical thing to do!
