The ~24-hour period of the circadian clock in mammals and other species requires a time delay that is built into a transcriptional negative feedback loop. The mechanisms of this time delay have been unclear, but these authors now provide evidence that microRNAs have an essential role. Using cells and mice that are deficient for the Dicer gene, they show that microRNAs generate a time delay by slowing down the translation of the period circadian clock 1 (PER1) and PER2 proteins, which are key components of the clock.