Calendar Man is bestowed by Mr. Myxlplyx with a magic hat that lets him see through the other comic panels. Using this new ability, he discovers that Bruce Wayne is Batman and proceeds to attack him and Superman. Not used to his new powers, he accidentally severs his own arm and gets saved by the Dynamic Duo. Mr. Myxlplyx then restores his arm back and erases his memory of the event.
From – Batman/Superman Vol. 2 #22