  Modeling most of the tests of general relativity requires to know the
function relating light travel time to the coordinate time of reception and to
the spatial coordinates of the emitter and the receiver. We call such a
function the reception time transfer function. Of course, an emission time
transfer function may as well be considered. We present here a recursive
procedure enabling to expand each time transfer function into a perturbative
series of ascending powers of the Newtonian gravitational constant $G$ (general
post-Minkowskian expansion). Our method is self-sufficient, in the sense that
neither the integration of null geodesic equations nor the determination of
Synge's world function are necessary. To illustrate the method, the time
transfer function of a three-parameter family of static, spherically symmetric
metrics is derived within the post-linear approximation.
