Let there now be another observer B who travels in the x direction from (0,0,0,0) for 5 years of A -coordinate time at 0.866 c to (5 years, 4.33 light-years, 0, 0). Once there, B accelerates, and travels in the other spatial direction for another 5 years of A -coordinate time to (10 years, 0, 0, 0). For each leg of the trip, the proper time interval can be calculated using A -coordinates, and is given by Δ Δ τ τ l e g = ( 5 years ) 2 − − ( 4.33 years ) 2 = 6.25 y e a r s 2 = 2.5 years .