That is an issue because in general, at low speeds, there cannot be differences in the counting of clocks.
Once the clocks have been set in sync while overlapping there is no reason for them be out of sync of that quantity when they are at a distance.
That term is simply a time. You should know...
Yes, low speed and short distances indeed.. but only low-speed v/c<<1 is involved for a classic approximation to be found. There is something ill posed on assuming also short distances
Maybe you want to suggest that t'=t-vx/c2
( first order Lorentz transformation) should be chosen instead of...