You can specify a time constraint in Messages in a Sequence diagram. Simulation Toolkit uses the time constraint as a timestamp on a Message when it records simulation as a Sequence diagram(see Executing an interaction model). During . During simulating an interaction, Simulation Toolkit checks if the active States contain State contains the required State , (State Invariant), and and the time is exactly as exactly specified in the time constraint but not delayed specified (Time constraint). However, the time constraint is not used as the delay time.