I didn't quite understand how to apply a time shift operator on a system when checking time invariance. For instance, if we have the system: I{x}(t) = (a(t)) and I want to shift it 7 units forward; would we get this: " Vac} (t) = (a(t+ T) ) + or does the time shifted expression equal: (ac ( t ) ) +T ? Is there any intuitive explanation for this? I guess what I'm asking in a way is whether the system's time changes the signal's time