This article explores the widely reported finding that the subjective duration of a stimulus is positively related to its magnitude. In Experiments 1 and 2 we show that, for both auditory and visual stimuli, the effect of stimulus magnitude on the perception of duration depends upon the background: Against a high intensity background, weak stimuli are judged to last longer. In Experiment 3 we show that the effect of intensity becomes more pronounced at longer durations, consistent with the idea that stimulus intensity affects the pacemaker component of an internal clock, and that it is the difference of a stimulus from the background, rather than its absolute magnitude, which influences the rate of the pacemaker. These results urge a modification to the oft-repeated claim that more intense stimuli seem to last longer, and provide an important constraint on any model of human timing. (Contains 4 figures.)