The circadian clock is controlled by a transcription–translation circuit that revolves around the transcription factors CLOCK and BMAL1. Chromatin modifications are important for maintaining circadian rhythms, but their precise role and how they are regulated remains unclear. DiTacchio et al. find that the histone demethylase JARID1A is recruited to the promoter of the Per2 gene (which encodes a clock component) through the action of CLOCK and BMAL1. They show that JARID1A enhances CLOCK–BMAL1-dependent transcription from the Per2 promoter and that, in its absence, Per2 transcripts oscillate with a shorter period. This function of JARID1A did not require its demethylase activity. Instead, JARID1A promoted histone acetylation, possibly by inhibiting the activity of the HDAC1 deacetylase. The exact mechanism by which JARID1A mediates these effects on the clock remains to be determined.