At some point will ISO-8601 have to be updated to include a field for difference in speed of time?
https://github.com/withinboredom/time
From the earth frame of reference, the lunar day is different than from a lunar frame of reference.
GPS wouldn't work without accounting for relativity, for example.
Unlike "sometimes a second is longer than a second" (not literally true but it makes some sense in the context of relativity), this one just seems like a tautology to me.
In other words, the only thing we can say without qualification is that a second is just a second in the same frame of reference. All other statements must be heavily qualified.
Even things like "A's second is longer than B's" are only valid in some frames of reference and not others.
In the twin paradox thought experiment, one of the twins really has aged slower than the other (or, from their point of view, the entire earth has aged faster than themselves).
In that sense, relativity has effects more tangible than distortion of observations across a large distance.