The start of Modified Julian Date.
Mostly used by astronomers and astrophysicists, the Modified Julian Date (MJD) is a dating method that is defined by subtracting 2,400,000.5 days from the current Julian date (JD), which is calculated by counting the number of days past since Noon January 1, 4713 B.C.E. The MJD gives the number of days past Midnight November 17, 1858. MJD was first used in 1957 by scientists at the Smithsonian Astrophysical Observatory to track satellites.