Subjects listened to a series of musical selections and then judged the duration of each selection. Some subjects were informed beforehand that timing was involved (prospective timing) whereas others were informed afterwards (retrospective timing). Half the groups performed a concurrent proofreading task during stimulus presentation. The results showed a trade-off between temporal and nontemporal task performance: prospective-timing groups were more accurate in judging time and were worse at proofreading, whereas retrospective-timing groups were relatively poor at judging time but better at proofreading. This pattern is consistent with Michon's notion of an essential equivalence between temporal and nontemporal processing, and supports the predictions of an attentional allocation model of timing. The proofreading task interfered both with prospective and with retrospective timing, and both types of time judgments were influenced in the same way by effects of stimulus context. These results imply that similar timing processes operate under prospective and retrospective conditions.