Module gregorian 

Gregorian calendar.

Implements the proleptic Gregorian calendar with Julian Day Number conversions, following the algorithms in Dershowitz & Reingold, Calendrical Calculations (4th ed., Cambridge University Press, 2018).

Historical Context

The Gregorian calendar was introduced by Pope Gregory XIII in October 1582 to correct the Julian calendar’s accumulated drift (~10 days by the 16th century). It refined the leap year rule: divisible by 4, except centuries not divisible by 400. This keeps the calendar synchronized with the vernal equinox to within 1 day per 3,236 years.

This implementation uses the proleptic Gregorian calendar, extending the Gregorian rules backward before 1582 for computational uniformity. Year 0 corresponds to 1 BCE in historical reckoning (astronomical year numbering).

Structs

GregorianDate

A date in the proleptic Gregorian calendar.

Enums

GregorianMonth

The 12 months of the Gregorian calendar.

Constants

GREGORIAN_EPOCH_JDN

Julian Day Number of the Gregorian epoch: January 1, 1 CE (proleptic).

Functions

gregorian_is_leap

Whether a Gregorian year is a leap year.

gregorian_to_jdn

Convert a proleptic Gregorian date to a Julian Day Number.

gregorian_year_days

Days in a Gregorian year (365 common, 366 leap).

jdn_to_gregorian

Convert a Julian Day Number to a proleptic Gregorian date.