1
general::
Phrase(s): *different as night and day
Cliché completely different. (*Also: as ~.) • Although Bobby and Billy are twins, they are as different as night and day. • Birds and bats appear to be similar, but they are different as night and day.
McGrawhill's American Idioms And Phrasal Verbs
2
general::
Also, different as day and night. Totally unlike. For example, Although they are sisters, they are as different as night and day. Also see NIGHT AND DAY.
American Heritage Idioms