What are Julian Date and Modified Julian Date (MJD)?
A Julian Date is the interval of time in days and fractions of a day, since Greenwich noon, January 1, 4713 B.C. This was introduced by astronomers to provide a single system of dates that could be used when working with different calendars. The Julian Date for the day beginning at UTC noon, January 1, 2010 was 2455197. For that day at 1800 UTC, the Julian Date would be 2455197.25. Because the Julian Date is such a large number, the Modified Julian Date (MJD) is often used because it references a much more recent epoch. MJD is the Julian Date minus 2400000.5, which is equivalent to the number of days elapsed since 17 November 1858. The half-day difference adjusts the outcome so that the MJD references the modern day boundary of midnight instead of noon. Using MJD is useful for determining the interval between two days without involving calendar dates. The MJD for January 1, 2010 was 55197.
