Most theorists propose that when a person is aware that a duration judgment must be made (prospective paradigm), experienced duration depends on attention to temporal information, which competes with attention to nontemporal information. When a person is not aware that a duration judgment must be made until later (retrospective paradigm), remembered duration depends on incidental memory for temporal information. In the present article we describe two experiments in which durations involved with high-level, executive-control functions were judged either prospectively or retrospectively. In one experiment, the executive function involved resolving syntactic ambiguity in reading. In another experiment, it involved controlling the switching between tasks. In both experiments, there was a unique cost to the operation of control high-level, executive functions which was manifested by prospective reproductions shortening a finding that supports an attentional model of prospective timing. In addition, activation of executive functions produced contextual changes that were encoded in memory and resulted in longer retrospective reproductions, a finding that supports a contextual-change model of retrospective timing. Thus, different cognitive processes underlie prospective and retrospective timing. Recent findings obtained by some brain researchers also support these conclusions.