Sartre fits fully within the phenomenological tradition inaugurated by Husserl, although he somewhat reelaborates it in an original way, on the basis of Heidegger's philosophy, with the aim of outlining, in a first stage of his thoughts dating back to the publication of Being and Nothingness (1943), the features stemming from his peculiar atheistic existentialism. Subsequently, in the mature stage of his intellectual itinerary, Sartre will attempt to combine the existentialist ideas with the basic principles of Marxism, a synthesis that will create important works such as Search for a Method (1957) and Critique of Dialectical Reason (1960). This chapter analyses time from the phenomenological perspective of the Sartrean ontology of temporality. This analysis allows to conceive the typically human "existential time" as a permanent existence out of oneself. For this purpose, we will retrace the fundamentals of Sartre's phenomenological ontology outlined in his most important work of 1943, Being and Nothingness, focusing in particular on the structures of consciousness understood as "être-pour-soi" ("being-foritself"), i.e. the conditions of man's "absolute freedom", which Sartre refers to when he shows the one "pour-soi" as the being who is pure nothingness.