Daily growth increments and monthly markings on Silurian and Devonian corals and brachiopods were counted using a maximum count method. Early and Middle Silurian fossils indicate that the number of days per year during these periods was 421 and 419, respectively; the number of days per year in the early Middle Devonian Period was 410. The number of days per month has decreased from 32.4 in the Early Silurian to 31.5 in the early Middle Devonian. The values obtained for these periods are considered more accurate than the values obtained using average count methods.