What is the official Harvard style?

“Harvard style” is a popular name for the author-date type of parenthetical referencing.

This way of referencing does not originate from Harvard University¹⁾. There are many variants of “Harvard style” in use, and there is no “official” Harvard Style. Therefore, you should either consult the style guide of your own institution or of your professional association.

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¹⁾ Anecdotal evidence suggests the name originates with an English visitor of the Harvard University Museum of Comparative Zoology. See Chernin, Eli (1988). "The 'Harvard system': a mystery dispelled", British Medical Journal. October 22, 1988, pp. 1062–1063.