Universal calendar

The universal calendar is a timekeeping system used to unambiguously date events in the Garden universe, and specifically the history of the planet Winter.

Definition
The universal calendar counts days from a semi-arbitrary zero point, the Great Syzygy.

Zero point
The zero point of the universal calendar is defined as the precise moment of conjunction of the Great Syzygy, an astronomical event in which the Sun, the planet Winter and its two moons were in alignment on Winter's winter solstice. This event is taken to have occurred at 00:00, midnight, in universal time, making the first 24 hour period following this moment day 0 of the universal calendar. Winter's prime meridian, that is the line of zero longitude, is also defined as the line on which it was actually solar midnight at this moment in time.

This choice of date simplifies the conversion and comparison of dates in different calendar systems, using commonly recorded astronomical events to compare to extrapolate the time passed since the reference time. For example, since Bigmoon was perfectly full at exactly midnight on day 0, its synodic period of 64.1647 days can be extrapolated to find any day and time in history at which Bigmoon was directly oppositely aligned with the sun and thus perfectly full: day 64, 03:57; day 128, 07:54; day 192, 11:51; etc.

Year counting
For ease of referring to dates, the universal calendar is divided into years which each have some number of days, thus any given day can be said to fall in a particular year. The length of Winter's orbital period, ~251.801612 days, is approximated by the following year counting system:


 * The first year following the zero point, year 0, has 251 days.
 * If a year is a multiple of 625, it has 252 days.
 * Otherwise, if it is a multiple of 5, it has 251 days.
 * Otherwise, it has 252 days.

This system has an average year length of 251.8016 days, which is close enough to the actual value as far as recorded history is concerned, and ensures that the winter solstice always falls on the first or second day of any given year.

Similar systems of timekeeping have been employed by people on Winter. For example, solar calendars used in Oksandanderi use a system of 252-day years with each fifth year having 251 days, and in the Tyon-Arat the remaining 0.0016 day discrepancy was also accounted for by an extra day every 625 years. Note that the particular years chosen to have days added or removed do not necessarily align with the universal calendar.

AGS and BGS
Negatively numbered years are commonly referred to as BGS, while non-negative years are AGS, for before and after the zero point. Year 0 is called 0 AGS, with 1 BGS immediately preceding it. GS might stand for "Great Syzygy" or "Garden start", considered an arbitrary "start" of most recorded history.