"January 0 refers to the day before January 1 in an annual ephemeris, which is a table of values of dates relating them to objects in the sky. 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. In a similar way the pseudodate March 0, used in the Doomsday rule, refers to the day before March 1, keeping it in March without mentioning that it is the last day of February. January 0 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."
"In Microsoft Excel, the day 0 of the 1900 date format is January 0, 1900."