intercalary days

As the calendar winds down and makes ready to welcome a new year, we pause to take a look at a few non-standard dates, evoked for neat calculation and exceptional circumstances. 0 January is the manner for referencing the coordinates of astral bodies—used in tables for stellar navigation and astrology—the day before the start of a calendar year while still keeping the annual ephemeris inclusive.
Furthermore, for practical purposes, the epoch of computing and programming only reaches back to 1900—and though they had intended the starting point of 0 January to be New Year’s Eve 1899, because the year 1900 was erroneous reckoned as a Leap Year (it is a Common Year under the Gregorian Calendar but a Leap Year under the Julian system, in use in some jurisdictions until 1923) 0 January 1900 is actually the penultimate 30 December 1899. While most of Western Europe transitioned from the Julian to Gregorian calendars by excising a week of Sundays—not at all in a coordinated effort either—the Swedish Empire, seeing hardship elsewhere, announced it would gradually catch up, by phasing out leaps days over the following four decades—from 1700 to 1740. Conflict and conquest, however, made keeping an accurate count of cheat days difficult and at one point—in 1712—Sweden observed 30 February. Ultimately, in 1753, and despite the earnest efforts of civil servants calendar synchronisation was complete, by fast-forwarding from 17 February to 1 March.