This paper argues that Einstein’s conventionalist definition of time is sufficient for, but not necessary to the geometric modelling of Special Relativity. A different convention allows that any time interval t, can be measured by dc, the distance travelled from an origin by the spherical wave-front of a light pulse, c. This means that the relationships represented by the hyperbolic geometry of Minkowski can also be represented by circular function geometry (CFG), where the spherical surface of c provides both a fourth set t, of frame-dependent co-ordinate points and a parameter s, for measuring intervals that are invariant between reference frames. However, sine values under the circle range from 1-0, rather than 1-∞. This does not allow that for a reference frame velocity ≈ c, any interval length ≈ ∞. Furthermore, since CFG does not subdivide space-time into past and future zones, it excludes the possibility of backwards time travel for signal velocities > c
