This paper deals with the enforcement of generalized mutual exclusion constraints (GMECs) on time Petri nets (TPNs) with uncontrollable transitions by restricting the firing intervals of controllable transitions. Existing approaches do not exploit the timing information and consequently the system permissiveness is limited. The key idea behind the proposed approach is the online computation of a graph representing a reduced portion of the state space of a TPN system, and precisely the states that can be reached from the current one by firing only uncontrollable transitions. Such a graph is called partial modified state class graph (PMSCG) and is derived from another graph recently presented in the literature. Based on the PMSCG, a procedure to compute a supervisory control law enforcing a GMEC on a TPN system in a maximally permissive way is presented.