How people perceive temporally overlapping intervals can inform us about the architecture and constraints of the human timing system. In the present study, we examined the time perception of two overlapping intervals in a nested context. In this context, one short interval (1 s) was temporally nested within another long interval (3 s). The data showed that although participants' perception of the short interval was unaffected by its temporal position within the long interval, estimates of the long interval decreased, the later the short interval appeared. These data indicate that participants perceive two overlapping intervals as three segments that must be summed in order to estimate the long interval. Importantly, the temporal relationships between overlapping intervals affect the estimates, because a recency weighting is applied to each segment during the summing process. Within pacemaker-accumulator models, these results could be seen as supporting a timing system composed of a single pacemaker and a single accumulator, but they could also constrain any account of human interval timing.Keywords Time perception . Multiple timing . Reproduction . Overlapping intervals . Pacemaker-accumulator models Many cognitive models that aim to explain how we perceive, produce, estimate, and compare time intervals have posited that an internal pacemaker emits pulses at a certain rate (linearly or nonlinearly; Wearden & Jones, 2007) and that these are collected by an accumulator (e.g., Treisman, 1963). The number of pulses can be stored and then used; for example, in an interval reproduction task, the participant can cease the reproduction when the same number of pulses has been accumulated as were perceived. Although these pacemakeraccumulator models of interval timing are very successful at describing how we perceive and estimate simple intervals (Rammsayer & Ulrich, 2001), it is unclear how they can be applied to the timing of multiple overlapping intervals. In this respect, an important question is whether overlapping intervals are timed by a single pacemaker and accumulator, or whether separate pacemakers and/or accumulators are dedicated to each interval.To investigate this question, in a previous study we asked participants to estimate two intervals of the same duration that overlapped to a lesser or greater extent (Bryce, SeifriedDübon, & Bratzke, 2015). Predictions were derived from various timing models, defined by the number of pacemakers and accumulators available to the timing system (see also van Rijn & Taatgen, 2008). Across three experiments, the estimate of the second interval consistently increased as the intervals were more temporally separated, whereas the estimate of the first interval was either unaffected or decreased. Overall, a model constrained by a single pacemaker and a single accumulator with an additional weighted calculation feature (referred to as the SPSA weighted model) best explained the results. This model treats two overlapping intervals as three segments (composed of the first ...