Objective: The nature of time-processing alterations in ADHD was assessed by means of duration judgments and temporal set-shifting tasks lasting several seconds and milliseconds. Method: After training with visual sample stimuli for long and short durations, 31 children with ADHD and 29 controls estimated the durations of test stimuli. During testing, the temporal context was systematically varied by shifting the duration of stimulus sets to longer or shorter intervals. Results: Children with ADHD generally overestimated the durations of stimuli on the seconds scale. Their assessment of stimuli on the milliseconds scale can be characterized as less-efficient adaptations to new temporal sets alongside otherwise normal discrimination performance. Conclusion: Findings support a pure time perception alteration in ADHD. In addition, results provide first evidence that difficulties in mental set-shifting, which have been reported for other tasks, extend to temporal processing in children with ADHD.