Time of Day Modified Julian Date https://www.commoncoreontologies.org/TimeOntology http://aa.usno.navy.mil/data/docs/JulianDate.php A Day begins at midnight GMT within the Modified Julian Date reference system. The Modified Julian Date (MJD) is related to the Julian Date (JD) by the formula: MJD = JD - 2400000.5 A Time of Day as specified according to the Julian Calendar using the Modified Julian Date epoch. MJD