Well comparing one second to another is like comparing one meter to another- they better all be the same by definition, otherwise measurement becomes meaningless. Varying quantities of time on a device does have impact- particles move, photons are exchanged, etc. At a small enough level, this all happens quite randomly- we would observe a number of very different states if we were able to examine this device very closely at several different "times". The point is that a second -just like a meter- is NOT absolute: one second of my time will not always be the same as one second of your time, and neither will the distance we measure between any two points we agree on.
Without going into existential philosophy on the nature of reality (that will muddle the content of this post), time as a dimension makes perfect sense. Take the state of our entire universe as it is right now- that is one instant of time dt (there are also, I think, theories that time is quantized- let's pretend it's not for now). One dt later, a number of things will have changed. As we move along this mathematically linear axis, we obtain some 4-dimensional function. Or, if it's easier, one can reduce our 3 spatial dimensions to 2- then, picturing whatever 3-d shape is most convenient, we can stack up "slices" of this 2-dimensional universe to obtain a 3-dimensional shape.
Just as displacement in space is a noticeable effect, I think displacement in time is, arguably, also noticeable. Going back to the graphical model, I can notice my displacement through time by remembering an older time, perhaps when I was in the same place. "x" number of minutes ago, I was sitting here and this white box on my screen was blank. Now, "x" minutes later, it is full of characters. Along the way, if we imagine increasing values of "t" as we go along, we would see me typing this message. The main difference is that time is not physically perceivable- I cannot simply look at something and say that it is a number of seconds long. And also, we can thus far only move in one direction through time.
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