The circadian clock and cell cycle as separate pathways have been well documented in plants. Elucidating whether these two oscillators are connected is critical for understanding plant growth. We found that a slow-running circadian clock decelerates the cell cycle and, conversely, a fast clock speeds it up. The clock component TOC1 safeguards the G1-to-S transition and controls the timing of the mitotic cycle at early stages of leaf development. TOC1 also regulates somatic ploidy at later stages of leaf development and in hypocotyl cells. The S-phase is shorter and delayed in TOC1 overexpressing plants, which correlates with the diurnal repression of the DNA replication licensing gene CDC6 through binding of TOC1 to the CDC6 promoter. The slow cell-cycle pace in TOC1-ox also results in delayed tumor progression in inflorescence stalks. Thus, TOC1 sets the time of the DNA pre-replicative machinery to control plant growth in resonance with the environment.