Fix bug: Node copy constructor

src/tests/mpi/cluster/models.hpp

@@ -238,7 +238,7 @@ public:
               typename common::DoubleTime::type /* e */,
               const common::Bag < common::DoubleTime >& /* msgs */)
     {
-      _sigma -= _last_time - t;
+      _sigma -= t - _last_time;
       _last_time = t;
     }