MicroRNAs (miRNAs) are small RNAs that regulate gene expression posttranscriptionally via the 3' UTR of target mRNAs and were first identified in the Caenorhabditis elegans heterochronic pathway. miRNAs have since been found in many organisms and have broad functions, including control of differentiation and pluripotency in humans. lin-4 and let-7-family miRNAs regulate developmental timing in C. elegans, and their proper temporal expression ensures cell lineage patterns are correctly timed and sequentially executed. Although much is known about miRNA biogenesis, less is understood about how miRNA expression is timed and regulated. lin-42, the worm homolog of the circadian rhythm gene period of flies and mammals, is another core component of the heterochronic gene pathway. lin-42 mutants have a precocious phenotype, in which later-stage programs are executed too early, but the placement of lin-42 in the timing pathway is unclear. Here, we demonstrate that lin-42 negatively regulates heterochronic miRNA transcription. let-7 and the related miRNA miR-48 accumulate precociously in lin-42 mutants. This defect reflects transcriptional misregulation because enhanced expression of both primary miRNA transcripts (pri-miRNAs) and a let-7 promoter::gfp fusion are observed. The pri-miRNA levels oscillate during larval development, in a pattern reminiscent of lin-42 expression. Importantly, we show that lin-42 is not required for this cycling; instead, peak amplitude is increased. Genetic analyses further confirm that lin-42 acts through let-7 family miRNAs. Taken together, these data show that a key function of lin-42 in developmental timing is to dampen pri-miRNAs levels, preventing their premature expression as mature miRNAs.