Abstract Many data acquisition systems used in meteorological field studies have digital clocks which indicate true time in hours, minutes, and seconds. This paper describes a circuit which adds the date to the time indication, automatically increments it at midnight, and outputs to the data logger. The Julian Day Indicator has operated without trouble during a five-month field measurement period. By providing a positive identification of each data scan it simplified computer reduction of the micrometeorological data. The parts required for the device can be purchased for less than $100 exclusive of power supplies.