Thank you for response.
I am using it in "Timer delay" property of "Timer Event" node.
This property supports expressions cause I can invoke #{service.calculateDate()} when service is declared
as local variable in process(for instance kcontext.setVariable("service", new SomeObjectService()))
BUT when service becomes global variable suddenly JBPM cannot invoke its method in "Timer Delay" of "Timer Event".
I suppose global variables are not visible in context of Timer Event (just local, process variables).
In our project we assumed that services will be provided to processes by global variables and work item handlers.