December 31  Index  January 1 
Every Year 
January 0 is an alternative name for December 31. 
Every Year 
January 0 refers to the day before January 1 in an annual ephemeris.
It keeps the date in the year for which the ephemeris was published, thus avoiding any reference to the previous year, even though it is the same day as December 31 of the previous year. 
Every Year 
January 0 occurs in the epoch for the ephemeris second, “1900 January 0 at 12 hours ephemeris time”. This entry needs to be expanded.

1 BCE  January 0, 1 BCE is the epoch for MATLAB.
MATLAB (matrix laboratory) is a numerical computing environment and fourthgeneration programming language. MATLAB allows matrix manipulations, plotting of functions and data, implementation of algorithms, creation of user interfaces, and interfacing with programs written in other languages, including C, C++, Java, Fortran and Python. In 2004, MATLAB had around one million users across industry and academia that came from engineering, science, and economics backgrounds. 
1900  January 0, 1900 (at Greenwich Mean Noon) was the epoch used by Newcomb’s Tables of the Sun.
Newcomb’s Tables of the Sun is the short title for “Tables of the Motion of the Earth on its Axis and Around the Sun” that appears on pages 1–169 of Tables of the Four Inner Planets (1895) by American astronomer and mathematician Simon Newcomb. The book contains Newcomb’s mathematical development of the position of the Earth in the Solar System, which is constructed from classical celestial mechanics as well as centuries of astronomical measurements. Most of the book, however, is a collection of tabulated precomputed values that provide the position of the sun at any point in time. Newcomb’s Tables of the Sun were the basis for practically all ephemerides of the Sun published from 1900 through 1983, including the annual almanacs of the U.S. Naval Observatory and the Royal Greenwich Observatory. The tables have not seen much use since 1984 when they were superseded by more accurate numericallyintegrated ephemerides developed at Jet Propulsion Laboratory, based on much more accurate observations than were available to Newcomb. Newcomb’s calculations also did not account for the effects of general relativity which was unknown at the time. Nevertheless, his tabulated values remain accurate to within a few seconds of arc to this day. Newcomb’s work was no small accomplishment, especially considering that it predated computers by more than a half century. His work emphasizes the tables, but the underlying theories are more important because the formulas he developed are still used in astronomical software and other computer algorithms. Newcomb developed similar formulas and tables for the planets Mercury, Venus, Mars, Uranus and Neptune. Those of the inner planets have proved to be the most accurate. 
1900  January 0, 1900 was the epoch for the Dublin Julian Date.
The Julian date is the continuous count of days since the beginning of the Julian Period—it is used primarily by astronomers. Because the starting point or reference epoch is so long ago, numbers in the Julian day can be quite large and cumbersome. A more recent starting point is sometimes used, for instance by dropping the leading digits, in order to fit into limited computer memory with an adequate amount of precision. The Dublin Julian Date is the number of days that has elapsed since the epoch of the solar and lunar ephemerides used from 1900 through 1983 in Simon Newcomb’s Tables of the Sun (1895) and Ernest W. Brown’s Tables of the Motion of the Moon (1919). This epoch was noon UT on January 0, 1900, which is the same as noon UT on December 31, 1899. The Dublin Julian Date was defined by the International Astronomical Union at their meeting in Dublin, Ireland, in 1955. 
1900  In Microsoft Excel and Lotus 123, the day 0 of the 1900 date format is January 0, 1900.
While logically January 0, 1900 is equivalent to December 31, 1899, these software packages do not allow users to specify the latter date. If the user enters “1/1/1900” in a cell formatted for Long Date, Microsoft Excel will display “Sunday, January 1, 1900” in the cell. If the user subtracts one day from that result, Excel will display “Saturday, January 0, 1900”. However, Excel will not recognize a userentered date of “1/0/1900”. On June 11, 2013, IBM announced the demise of the Lotus 123. 
December 31  Index  January 1 