Seconds don't vary, your approximation of a second varies.

If you want to nitpick, you're going to need to start with Cesium-133 oscillation periods as averaged from over nine billion samples.

I like the cut of your jib. But I suppose what I was saying was that, for the purposes of establishing “now”, “now” isn’t really standardizable except with respect to a single inertial frame, and any time-keeping that occurs outside of this inertial frame will not be absolutely in sync, even though it may have a highly accurate measure of time for its own inertial frame.

This matters when computers are bouncing records about time around satellites and across the global.

Edit: I have no idea what I’m talking about so please do not hesitate to correct me on the particulars!

Seconds do vary because there are no privileged reference frames in the universe. You'll get a different measurement if your cesium measuring experiment is traveling at a different speed than you are.

That's the direction of your mass-energy vector, not seconds.

A bit like complaining that UTC is wrong because it doesn't match your local Solar time.