We consider here time Petri nets (the TPN model) and two of its state space abstractions: the state class graph (SCG) and the zone based graph (ZBG). We show that only some time points of the clock/firing domains of abstract states are relevant to construct a TPN reachability graph. Moreover, for the state class graph method, the graph computed using relevant time points is smaller than the SCG. Fulltext Preview For further information, please visit this web site.