  A perturbative method to compute the total travel time of both null and
lightlike rays in arbitrary static spherically symmetric spacetimes in the weak
field limit is proposed. The resultant total time takes a quasi-series form of
the impact parameter. The coefficient of this series at a certain order $n$ is
shown to be determined by the asymptotic expansion of the metric functions to
the order $n+1$. To the leading order(s), the time delay, as well as the
difference between the time delays of two kinds of relativistic signals, is
then shown to take a universal form for all SSS spacetimes. This universal form
depends on the mass $M$ and a post-Newtonian parameter $\gamma$ of the
spacetime. The analytical result is numerically verified using the central
black hole of M87 as the gravitational lensing center.
