Scheme of the two-way time transfer. The signal leaves observer A at a coordinate time t_A1, corresponding to the proper time τ_A(t_A1) of clock A, is reflected from observer B at a coordinate time t_B, corresponding to the proper time τ_B(t_B) of clock B and τ_A(t_B) of clock A, and is finally received by observer A at the coordinate time t_A2, corresponding to the proper time τ_A(t_A2) of clock A.