In this paper, I analyze the verbal suffix -uŋil in Washo as an optional past tense. It is optional in the sense that it is not part of a paradigm of tenses, and morphologically tenseless clauses are also compatible with past time reference. Specifically, I claim that -uŋil is the morphological exponent of a tense feature [past], which presupposes that the reference time of the clause, denoted by a temporal pronoun, precedes the evaluation time. Meanwhile, morphologically tenseless clauses lack a semantic tense restricting the value of the reference time pronoun. In comparing this analysis with one containing a covert non-future tense in morphologically tenseless clauses, I show that the range of empirical contexts that distinguish these analyses is quite narrow. However, I offer a novel argument against a covert tense analysis based on the lack of Maximize Presupposition effects. Crucially, the fact that -uŋil does not form a paradigm of tenses results in a failure for Maximize Presupposition to apply. The proposed analysis places cross-linguistic variation at the level of the paradigm of tense features, namely whether they are present or absent, and if present, whether obligatorily so. This case study from Washo thus reveals what a language where tense features are optional can look like, and more generally contributes to the growing body of literature on cross-linguistic semantics devoted to uncovering the ways in which temporal interpretation can be achieved in natural language.