code: private void validateAndSetClock(long newSimulationTime) {
       
      // We do not use the getClock routine here as 
      // the Engine and JobTracker clocks are different at
      // this point.
      long currentSimulationTime = clock.getTime();   
      if (newSimulationTime < currentSimulationTime) {
        // time has gone backwards
        throw new IllegalArgumentException("Time has gone backwards! " +
                                   "newSimulationTime: " + newSimulationTime +
                                   " while currentTime: " + 
                                   currentSimulationTime);
      }
      // the simulation time should also match that in the engine
      assert(newSimulationTime == engine.getCurrentTime()) : 
      	     " newTime =" + newSimulationTime + 
      	     " engineTime = " + engine.getCurrentTime();
  
      // set the current simulation time
      clock.setTime(newSimulationTime);
    }
docs: Utility to validate the current simulation time
  @param newSimulationTime