If you think the title is clickbait, you're absolutely right. However, the contents of this thread are not. Scroll to tl;dr for full disclosure, if you're lazy or if you can't stand theoretical physics.
Otherwise, brace for some quality technobabble...
Some background
I won't delve too much into detail, because frankly this isn't a theory or even an idea (not that I would be capable of formulating one...), but rather musing on the topic of the nature of time. Now, I'm one of those popular science nerds who is fascinated by things like the recent black hole firewall problem, the applicability (or a lack thereof) of String theory, and the entire field of cosmology, all the while understanding almost none of the underlying mathematics. I do find this stuff important, though, and have done my share of reading - so what follows is not exactly simply pose or flat out sci-fi. Doesn't mean it's not stupid, though, so feel free to call me out on that.
Some more background
I recently watched this panel, which in and of itself isn't a very good hour and a half to spend. However, near the end Max Tegmark raises an interesting idea: that of time being an emergent property of the universe. More precisely he hints that if there is nothing to keep track of time in the universe, then does time really exist? Kind of like gravity - it's not really a thing on quantum scale, but only becomes a dominating factor on macroscopic scales.
This may sound like a dead end argument, but it doesn't take too much to realize that there's something here that is far more interesting. So let me start by laying down some groundwork and list three things more or less relevant to my musings below:
1) the idea of the many worlds theory, which essentially states that each time the wave function of a particle collapses, eg as a result of the particle being observed, the entire universe is split into more than one world, by implication suggesting that any and all possible universes exist in some larger realm. Now, I'm not really a fan of the many worlds interpretation, but since I'll be evoking quantum mechanics, it does become unavoidable in a sense
2) the notion of an inertial frame of reference, which in relativity is generally applied to the whole of spacetime as an observational paradigm
3) the Minkowski diagram, which essentially provides a handy visual aide in describing the relationship between motion through space and time within relativistic spacetime. In particular, it neatly describes how one relates to the other
The thing itself
My whole idea is that time pops into existence kind of like our universe within in the many worlds interpretation - however in this case if (and not when) an observer is present. This emergence of the passage of time does not explicitly occur in an inertial frame of reference, as an inertial reference frame does not interact with the system. That is, on a theoretical level the applicability of an inertial reference frame may not be as straightforward at all points of spacetime if it is important that the concept of time exist at each of these points. In practice this restriction does not apply in most cases as without a local observer there is nothing for the inertial reference frame to observe anyway. This latter is important, because it maintains the relevance and validity of an inertial reference frame whether or not passage of time is present.
An observer in this case might be a Cesium atom, which oscillates at a fixed frequency (as in an atomic clock). Or whatever the configuration and necessary nature of interactions of cohered particles is that is necessary for their states to decohere and some chunk of matter to take shape. Conversely, once the atom is split into its constituent fundamental particles, these cohere, and the observer, as it were, stops existing. On the Minkowski diagram the Cesium atom always exists in the valid region, inhabiting a specific point in time and space; however, once its constituents particles are able to cohere, the atom and hence any spatial relationship or notion of time "generated" by its prior state cease to be valid concepts altogether.
In a nutshell, in a quantum state time stops existing as a "thing" and will only re-emerge when a particle decoheres into an observable configuration. This does not violate causality any more than quantum physics already does, because a collapsed wavefunction is not necessary for the particle to move through spacetime. However, it is necessary for the particle to occupy a specific location and, in this case, the emergence of the concept of time.
Some conclusions
How is this relevant? Well, at the current age of the universe it really isn't. And it won't be for a very very long time. However...
1) it would still be the case that time as such might not exist at all in patches of spacetime that are exceptionally empty
2) in the earliest moments of the universe when everything was little more than non-interactive quantum foam, the absence of time (or rather the possibility of the absence of the notion of time) might become relevant as this tackles questions like "does time have a beginning?" and "did time start at the beginning of the universe?". The answer would be no in both cases.
3) it also becomes relevant in the later stages of the Big Freeze at which point black holes have decayed and the entire universe, as large as it may be, no longer contains anything other than virtual particles generated by dark energy and a dilute soup of subatomic remnants that where emitted into the grand void via Hawking radiation. At this point no more interactions occur, no particles are ever observed and hence never decohere. As such, time stops existing. This doesn't happen everywhere at once, but if nothing else out of the ordinary takes place (such as the Cosmological constant suddenly changing and universe choosing to collapse in on itself all of a sudden), will eventually happen everywhere
4) another place where this may or may not be relevant, is either within or at the event horizon of a black hole. With all the recent huzaa over what they really are and what really happens to stuff that enters a black hole, it's out of my league to speculate on. However, time would still stop being a thing at any point where matter is either ripped completely to shreds or becomes pure energy
tl;dr Now, like I said - I'm not a physicist, but rather simply find this stuff interesting. So without further ado, if nothing else, consider this post clickbait and fire away with your own best conspiracy theories as to what time is or isn't!
