January 0 is an alternative name for December 31.
In an ephemeris
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.
January 0 also occurs in the epoch for the ephemeris second, "1900 January 0 at 12 hours ephemeris time". 1900 January 0 (at Greenwich Mean Noon) was also the epoch used by Newcomb's Tables of the Sun, which became the epoch for the Dublin Julian day.
- "Leap Seconds". Time Service Department, United States Naval Observatory. Retrieved 2006-12-31.
- ASTROCLK Astronomical Clock and Celestial Tracking Program (page 110)
- "XL2000: Early Dates on Office Spreadsheet Component Differ from Excel". Microsoft. Retrieved 2007-12-08.
In the Microsoft Office Spreadsheet Component, the value 0 evaluates to the date December 30, 1899 and the value 1 evaluates to December 31, 1899. ... In Excel, the value 0 evaluates to January 0, 1900 and the value 1 evaluates to January 1, 1900.