biblatex-bath is an extremely well-written style and its author implemented a field format
datelabel to control the typesetting of the year in the bibliography. Together with
nameyeardelim it only takes two lines to wrap the year in the bibliography in parentheses.
It is worth pointing out that
biblatex-bath is a style specifically designed to implement the author-year (Harvard) style as interpreted by the University of Bath's Library (http://library.bath.ac.uk/referencing/harvard-bath). If you want to deviate from custom styles it is often a be a better idea to start from one of the standard styles instead. Some small changes are often simple, but some things may turn out to be more complicated than initially thought (and more complicated than in the standard styles). But since
biblatex-bath is a very well-written style the chances that you run into troubles are quite low.
For future readers: Note that this answer applies only to
biblatex-bath. The standard styles automatically have parentheses around the year in citations. If you want to change that, have a look at How to (properly) remove the parentheses around the year in authoryear style? (v3).