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There is time as an interval between two events but you could also state that time itself has a certain duration (it passes). Now in physics it is not always very common to see time as something that has a duration. '
So to make that statement solid one should find a way to work it out that it is an illusion but how can you do that?
Several philosophers have offered different ways of seeing that our intuition of time is not internally consistent, and may only be a construct for human interpretation, rather than something ultimately real.
McTaggart directly attacks the notion of time as an ongoing process, what he calls the A-series of events. His actual argument is a bit abstruse, but special relativity, to some degree, captures it better and backs him up. If there is no way to declare events simultaneous, then our notion of time 'unfolding' faces severe obstacles.
Kant points out in his second 'antinomy' that we do not have a firm grasp on the notion of time because we can logically prove that it both must and cannot have 'ends'. If we cannot get a handle on its basic topology, he proposes, perhaps we should back off from considering it too much of a fact.
Loschmidt's paradox suggests that the direction of the flow of time has no consistent basis because we keep finding that the principles of physics are essentially reversible. Boltzmann included this notion in his initial notions of thermodynamics, suggesting that we experience consistent time only because we are occupying a part of space relatively near an event of extremely low entropy.
In physics time is measured by clocks.
In the more refined version of the Special Theory of Relativity time is one component of the four-dimensional spacetime. One has a certain choice how to decompose spacetime into time and space. Hence the measured time depends on the choosen system of coordinates. There is no global time, each observer measures his own local time. But there are rules how to convert the different results.
But as you write, a term like "duration of time" is not employed in physics.
It is discussed in the philosophy of physics whether time flows or whether we move through a preexistent 4-dimensional spacetime. See "Chapter 5. The Frozen River. Does Time Flow?" from Greene, Brian: The Fabric of the Cosmos (2004)
My question: Who does speak about a duration of time?
Special Relativity implies that time is an illusion, as Einstein said, but that causality is not: two events may be 'simultaneous' - when there is a reference frame in which the two time coordinates are equal - or not, in which case one event occurs before the other in all reference frames. The point is that this just refers to pairs of events; time in the classical or intuitive sense is just a coordinate, but events may be causally ordered (see spacetime interval). Moreover, in standard cosmology we have the cosmic time...
Essentially, time may be understood as something that 'passes' because it only makes sense if there are 'clocks'.
It is possible to work out that time is an illusion in the sense that it is not fundamental but is a function of mind.
Hermann Weyl makes the argument in his book 'The Continuum', as does Nagarjuna in 'Fundamental Wisdom'. If we look closely we find that when we assume time is truly real many paradoxes arise, and this is what allows us to work out that it is not.
Just as you say, "Time [is experienced] as an interval between two events". If you eliminate movement, you eliminate time. Therefore, if you are in a complete void or a world of no movement, the illusion of time is broken. Conversely, the illusion of infinity is broken once something changes. A theoretical, but scientifically grounded example is the concept of universal entropy. It is a perfect example of timelessness as the thermodynamic free energy of the universe dissipates and even the movement of atoms stop.
Another method is to consider the effects of time dilation and compression as objects approach the speed of light. If time is experienced differently when you are moving at immense speeds versus normal speeds, then it is an illusional construct because it cannot be two separate times simultaneously. It is in fact a relative experience, bringing us back to the theory of stillness being equal to timelessness.
