Several people objected to the notion of "simultaneity". Others replied
that simultaneity was OK in this case, because it didn't involve
simultaneity-at-a-distance.
Let me go even further: Simultaneity is a REQUIRED part of any physical
conservation law. Suppose some energy just disappeared, and your buddy
said "Oh, don't worry, it will come back
eventually." Yecccchhhhh! That's not a conservation law. Real physics
involves what we call *local* conservation laws, local in time and space.
Energy and momentum are conserved right here, right now.
Charge is conserved right here, right now.
Lepton number is conserved right here, right now.
Et cetera.
You can write separate and equally-valid conservation laws for each point
in space and each moment of time.