To check the skeletons of corals were taken, who died in the Devonian period. The number of ridges in their tree rings turned out to be in the range 385-400. These figures are close to the theoretical, calculated 396 days of the year in the Devonian period (when the length of the day equals 21 hours). But if the length of the year is possible to determine to what age are fossil corals, maybe the opposite: fossil corals to determine the time of the formation in which they are found. The prospect is very attractive. And if further tests show that the hypothesis of "coral clocks" is correct, geologists and oceanographers, finally, would be in the hands of a simple and reliable method of dating the layers.