For a twin paradox scenario, let there be an observer A who moves between the A -coordinates (0,0,0,0) and (10 years, 0, 0, 0) inertially. This means that A stays at x = y = z = 0 for 10 years of A -coordinate time. The proper time interval for A between the two events is then Δ Δ τ τ A = ( 10 years ) 2 = 10 years .