proximus

Proximus, proxima, proximum: the superlative of the adverb prope, with propior, propius as the comparative in between. There is no positive adjective; the series starts with an adverb and works upwards. "Nearest to" takes the dative in prose (proximus mari) and the accusative rather more in poetry and later Latin.

The real difficulty is time, not case. Proximus points at whichever neighbouring period the sentence is facing, so proximo anno is "next year" in a future context and "last year" in a past one, and Latin sees no ambiguity where English demands two different words. As a noun, proximus is your neighbour, which is the word behind dilige proximum tuum. English proximity, approximate, proximate.