"Simultaneity" is a word used in Science and has two distinct meanings. As usual, the meanings get conflated and lead to argumentation.
A) Perceptive simultaneity (Einstein's)
B) Time Zoning (Modern Scientism)
Einstein was referring to simultaneous events, he was referring to (A)
wherein two observers might report whether events were simultaneous
differently from each other due to the time that it takes for the light
to travel from one point to another giving the impression that an event
occurred later than it did.
In modern Scientism, the word
"simultaneity" refers to a time zone labeling of space for time
reference. When using this version of simultaneity what is being
referred to is the terminology or semantics to be used when referencing
what time something occurred. When it is 8:00 in New York, it is 5:00 in
California. Of course this has nothing to do with the behavior of the
physical universe, "physics". It is merely a labeling standard. The
standard is that you are to project a light photon from a point of
interest at any particular time of day and label every point the photon
reaches as being that time (of day) when it reaches that point, which of
course will be later.
The problem has arisen that the semantic
issue of time zoning is being taught in universities as a principle in
physics that explains things like special relativity. An oversight in
language is being used to justify why a theory is to be accepted. The
Lorenz equation is often called up as a authoritative reference to
justify the semantics. But in reality, the Lorenz equation had nothing
to do with time zone labeling as is often taught (and even represented
on Wiki as a physics principle).
Time itself on the other hand,
has nothing to do with labeling nor perception. Time is a measure of
relative change. It doesn't matter what you choose to call anything. The
only issue relevant to time itself is how much change occurred in one
event compared to another. Einstein referred to this as the comparison
of two clocks, "how fast one clock is running compared to another".
Special and general relativity deal with this time issue of comparisons
in rates of changes, not what labels are being used to refer to them.
when someone mentions "simultaneity" it might be wise to ask which
version they are referring to and see if it is actually relevant to the
discussion. Often people say that reality is merely whatever it is
called rather than what it actually is (a confusion of Aristotle dialectics).