[im3w1l]

Send three photons A B C. They arrive at times ta, tb, tc. Compute fraction (tc - tb) / (tb - ta). This can encode any positive real number with arbitrary precision. But clearly you need either very precise measurements or send the photons at a very slow rate.

[acchow]

Special relativity will give limits on how aligned the clocks can be (which should be a function of the distance between the clocks). So there will be precision limits.

[LegionMammal978]

Unless the clocks are accelerating, that shouldn't change the ratio between the two time differences, right? If the two ends have a constant velocity relative to each other, then each should perceive the other as being off by a fixed rate.