Two experiments examined whether time-based prospective memory performance is influenced by the continuous or discontinuous nature of an ongoing activity. The first experiment demonstrated that prospective memory performance was not influenced by the engagement in continuous or discontinuous ongoing activity. The second experiment demonstrated that a discontinuous ongoing activity negatively affected prospective memory performance when participants had to execute two time-based tasks for which the retention intervals partially overlapped. The results suggest that when individuals are engaged in multiple time-based tasks, a general timing disruption occurs, with a proactive interference effect resulting in costs that are detrimental to prospective timing. Copyright © 2009 John Wiley & Sons, Ltd.