An adapter includes a plurality of individually separable collars that allow for easy, on-site adjustment of the length of the adapter based on a measured offset of an outlet of a water supply assembly to which the adapter is attached. An exterior of each collar is configured to sealingly engage the outlet of the water supply assembly, such that the adjustment to the length of the adapter does not compromise the ability of the adapter to be sealingly coupled to the water supply assembly. A slip fitting provided along a downstream end of the adapter allows the plumbing fixture to be installed flush against a wall in a desired orientation, without compromising the sealing engagement between the adapter and either of the plumbing fixture or outlet opening.