Can you elaborate on this a bit? Would you say that gravity exists?
Time appears in relativity as a feature of our universe which can be measured and can be altered via motion. To our knowledge, it isn't a particle or a wave, but is a property which can be quantified, measured and observed objectively, if relatively. Time, in conjunction with the three spacial dimensions, provides the physical framework upon which all other physical theories are described.
I understand how it can be a grey area, which is why I was hoping you could give your criteria for "existence".
michabo, these are good questions, and I have to admit that I can not give you a clear line between about what I would say "it exists" and would not say "it exists".
I can think of several different lines (each of which again would be not clearly determinable for me), depending on the subject and the purpose of the context and the conversation.
If that´s what everybody would like, I would have no problem calling everything (including concepts, processes, thoughts, the past, the future, you-have-it) into that which exists. a definition by which god undeniably exists, for example.
Whilst if the mechanic asks me "does there exist a 10´´ key?", I am assuming that he doesn´t refer to the existence of potentiality, the past, the future or something, and that long elaborations about the existence of the potential for this tool to exist somwhere in the time/space continuum equals inviting physical contact.
A certain definition of a term limits the usability for certain purposes. One of my concerns in this thread was that including concepts like "potential" and the "past" into that which exists, the definition clearly left the frame of reference suggested by the topic, and that this very broad definition was introduced in order to prepare invalid equivocations. IOW: for me, we can include "potential" into that which exists, but certain conclusions from "it exists" that would be possible if we would apply a stricter definition, won´t be valid anymore.
My second concern is redundant overcomplication in the use of language.
"something moves". ok. we all know what is meant. "does movement exist?" adds a nominalisation and an abstraction at the same time. I am already beginning to doubt that this question and the intended answer is - in terms of usefulness - in any way proportional to the verbal effort. "does this thing move?" or "do things move?" is a more interesting question, and the amount of abstraction used suggests itself as adequate.
Once we find a way of answering "does movement exist?" with "yes", we can - with the same will to add another layer of abstraction - ask "does the existence of movement exist?", and then "does the existence of the existence of movement exist". i am not exactly sure at exactly which point in this sequence the line from abstraction to absurdity is crossed, but I suspect that "Does movement exist?" is already beyond the line.

"Does something potential exist? Does the potential of something exist? Does the existence of the potential of movement exist?", and even more urgent "Does the past exist?", "Does the existence of a potential past exist?", "Does non-existence exist?"
Let me put it this way: I have been engaging long enough in philosophical and abstract discussion to know when there is a concept behind a complex wording, and when sentences merely help sophistry and semantic trickery. Intuition, if you will. Not really a compelling parameter and not really a good argument, but I think in the end it comes down to this: intuition.
Thus, if you ask me "does gravity exist?" let me first ask you what purpose you have in asking this question, and which conclusions you are about to draw from the answer. Is it just another way of asking "Do you think gravity is a sufficiently accurate concept?" or are you - I am exaggerating hopelessly - going to conclude that because babies exist and gravity exists babies and gravity are basically the same; time exists, too, and hence is also basically the same, and since killing babies is murder killing time must also be murder.
You get the idea: "Does X exists?" in itself is not a particularly meaningful and useful question, imo, and at best it becomes useful by the purpose for which it is asked and meaningful by the context in which it is asked. I suspect that there is always a better way of communicating the actual question than by asking "Does X exist?".