Abstract: A truly concurrent and timeless semantics is proposed for a composition of sequential, non-deterministic processes with asynchronous communication. It is shown when this semantics differs from simple interleaving. Implementation-dependent time constraints determine a subset of all computations of the timeless semantics. This set is precisely characterized for given constraints. It is shown how the same set may be generated by a timed transition system, i.e., how to simulate time-constrained and concurrent computations by sequential, non-deterministic system. The paper includes a proof of the correctness of such simulation.