The cosmic chronometer method estimates H(z) directly from age differences of passively evolving galaxies, offering a route to the expansion history that is independent of integrated distance indicators and the sound horizon scale (Jimenez & Loeb 2002; Moresco et al. 2012). Current H(z) datasets from chronometers tend to favor a lower H0 and a slightly different late-time expansion history than that inferred from local distance ladders, while being sensitive to substantial systematics from stellar population synthesis, star-formation histories, and metallicity, so within ΛCDM one must finely control uncertain galaxy-evolution physics to keep these chronometer results fully consistent with both Planck CMB and SH0ES-style measurements (Moresco et al. 2016; Moresco et al. 2020).