We establish improved uniform error bounds on a second-order Strang time-splitting method which is equivalent to an exponential wave integrator for the long-time dynamics of nonlinear Klein--Gordon equation (NKGE) with weak cubic nonlinearity, whose strength characterized by $\varepsilon^2$ $0 < \varepsilon \leq 1$ dimensionless parameter. Actually, when \ll 1$, NKGE $O(\varepsilon^2)$ nonlinea...