Yes, I would never ever recommend that anyone ever sneak into his office only to flip a calendar or pull some silly prank. How did he think of doing that? And then to tell so many people means that everyone would be well informed by now of that. And we already know well that the man is such a wicked evil unforgiving person who spits poison whenever possible, so we are all bending over backwards to avoid him at all costs and to leave him completely uninformed of our whereabouts and doings, and you are completely not helping us with that.
But my sister always was a proficient prankster in her wayward youth so it makes sense that she would have passed that along.