This equation gives you the Reduced Julian Day (RJD) when you input a Gregorian (standard modern calendar date).
The RJD's starting point epoch is noon on November 16, 1858.
If you input January 1, 2000, this equation will return the Reduced Julian Day: 51545.0
If you input November 16, 1858, this equation will return the Reduced Julian Day: 0.0