Many stimuli induce short-term increases in the cytosolic concentration of free calcium ions ([Ca(2+)](i)) that encode signaling information about diverse physiological and developmental events. Slow cytosolic Ca(2+) oscillations that span an entire day have also been discovered in both plants and animals; it is thought that these daily Ca(2+) oscillations may encode circadian clock signaling information. A recent study focusing on the characterization of the extracellular Ca(2+)-sensing receptor (CAS) has provided insight into the molecular mechanisms by which the daily Ca(2+) oscillation in plants is generated. We summarize the major findings regarding daily oscillations of cytosolic Ca(2+) concentrations in plants and animals, and discuss hypothetical biological roles for the circadian clock-regulated physiology in plants.