[yor_on]

You could argue that there is no such thing as 'simultaneousness' as each frame present all other frames differently in four dimensions, when compared to each other. But then you also need to define how this would be possible, and as you do that find that the question above stays anyway. Because what you then comes to is still a mosaic, although now defined as each one having four dimensions slightly out of 'mode' with any other. And as each frame of reference will find unique definitions for all and any of the others frame of reference? Think about it.

I prefer the idea of 'time' as being somewhat of  a 'fabric' ) And the arrow as directly connected to lights speed in vacuum, meaning locally 'same' for us all when superimposing frames of reference. But then my 'time' also must become a conceptual description, as there is no way of measuring time, the (local) arrow we can measure though. If we had no way of doing so all locally measured speeds, and so 'motion' itself, would be imaginary.

(which it may well be:)
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Frames of reference are quite fascinating, and confusing, to me. All of this depends on how seriously you take the idea of direct measurements being what science rest on, and relativity naturally. And if we go back to virtual particles, all of this makes any idea of virtual particles connecting/communicating changes in 'points' of SpaceTime observer dependent. It's not enough defining the arrow as 'observer dependent' as I see it. You have to do the same with all four of our 'degrees of freedom' to make it fit.