Contrary to the lay theory that predicts duration knowledge of affective episodes to ameliorate negative experiences and weaken positive ones, we demonstrate that duration knowledge increases the extremity of affective experience. In experiments 1 and 2, participants either know the exact duration of the episodes or not and then experience either negative or positive episodes. The results show that, contrary to general intuition, duration knowledge worsens negative experiences and enhances positive experiences. In experiments 3a and 3b, we identify a boundary condition wherein the effect of duration knowledge is attenuated when participants focus primarily on the end of the experience (as opposed to the ongoing experience). In closing, we highlight the theoretical implications for studies on hedonic adaptation in general and the uncertainty effect in particular. Possible mechanisms for the effect of duration knowledge are discussed.