“Harvard Style” is a popular name for the (Author-Date) type of Parenthetical referencing.

It is not affiliated with Harvard University, and there is no 'official' Harvard Style.

Professional association styles of the (Author-Date) type should be consulted for an 'official' guide.

Author-Date referencing picked up the Harvard name when it was first used at the Harvard University Museum of Comparative Zoology in 1881. This article provides the historical details.