Tools Astrocalculator Julian Day module The Julian Day calculator module Julian Day This module converts between the calendar date, the Julian Day, and the Modified Julian Day. The Modified Julian Day is simply equal to the Julian Day - 2,400,000.5. To use the module, select which of the three dates will be the input, and then fill in its value. Then press the Compute button, and the corresponding values for the other two date systems will be displayed. Exercise: What calendar date does MJD = 0.0 correspond to?