Here’s a different way to describe that same method. Suppose that I send a signal to you and you immediately bounce it back to me. When the signal gets back to me, my watch reads 12:00:20 (independent of any synchrony rule). You need to set your watch so that the time it reads at the moment of bouncing is exactly half way between the time on my watch when the signal left (12:00:00) and the time when it got back (12:00:20)—that is, you should set it to 12:00:10. That half-way choice guarantees that we record the light as moving at the same speed on the way out as on the way back.
time dilation
time setting.
