Movement execution is not always optimal. Understanding how humans evaluate their own motor decisions can give us insights into the suboptimality. Here we investigated how humans time an action, and how well they can evaluate its outcome when synchronizing an arm movement with predictable visual stimuli. On each trial, participants had to decide when to start (reaction time) and for how long to move (movement duration) to reach a target on time. After each trial, participants judged the confidence they had that their performance on that trial was better than average. We found that participants mostly varied their reaction time, keeping the average movement duration short and relatively constant across conditions. Interestingly, confidence judgments reflected absolute deviations in reaction time, but were not related to movement duration. In two other experiments, we replicated these results in conditions where the contribution of sensory uncertainty was reduced. In contrast to confidence judgments, when asked to make an explicit estimation of their temporal error, participants' estimates were related to both reaction time and movement duration. Therefore, even though humans can evaluate both when they start their action and how long it lasts, they control the timing of their actions primarily by adjusting their reaction time, and they also rely on this reaction time for their confidence judgments.