The paper offers a new type of approach to the semantic phenomenon of adverbial aspect shift within the framework of finitestate temporal semantics. The heart of the proposal is a supervaluational concept of underspecification, and the idea of treating the meanings of temporal prepositions as dynamic presuppositions. The simple shifting algorithm used in the present approach derives the correct set of possible readings on the basis of lexical semantic input only, and, furthermore, may claim cognitive plausibility.