Module `calendrica`

High-level calendar and date API.

A date is an immutable value tied to a calendar, created with date or today. A calendar is given either by the name of a built-in one — a string such as "gregorian", "hebrew", or "islamic-umalqura" — or as a calendar object, from calendar(name) or made with calendar{…} for a calendar of your own. The built-in names are fixed: nothing can be registered under a name, so they always mean the same. A date exposes its calendar fields (year, month, day, …), its fixed Rata Die day number .rd, and its calendar object .calendar (with .name and .granularities), and can be re-expressed in another calendar with d:to(cal) (also d:as(cal) / d:date(cal)), which returns a new date.

The astronomical functions (sunrise, sunset, moonrise, moonset) return a moment — a date that also carries a time-of-day, adding .hour, .minute, .second, .zone (UTC offset), and the fractional .moment. A plain date has no time-of-day, so those read nil on it.

See the Manual for a guided introduction, and the Calendars and Holidays pages for the full lists.

Usage:

local calendrica = require("calendrica")
local d = calendrica.date({year=2024, month=4, day=23}, "gregorian")
local h = calendrica.date(d, "hebrew")   -- the same day in the Hebrew calendar
print(d, h)

Info:

Release: 0.2 2026-09-21

calendar (spec)	A calendar object: a built-in calendar by name, or a new calendar.
date (source, calendar)	Express a date in a calendar (a built-in name or a calendar object).
today ([calendar])	Return today’s date in the given calendar.
holiday (name, g_year[, calendar])	Return occurrences of a built-in holiday in a Gregorian year as date objects.
holidays (g_year[, calendar])	Return all built-in holidays occurring in a Gregorian year, sorted by date.
holiday_names ()	Return a sorted list of the built-in holiday names.
calendars ()	Return a sorted list of the built-in calendar names.
location (latitude, longitude, elevation, zone)	Construct a location for use with astronomical functions.
lunar_phase (date)	Lunar phase (0–360°) at midnight of date.
lunar_phases (phase, g_year[, calendar])	List of date objects on which the moon reaches lunar phase `phase` during Gregorian year `g_year`.
sunrise (date, loc)	Sunrise on date at `loc` as a moment, or nil if the sun does not rise.
sunset (date, loc)	Sunset on date at `loc` as a moment, or nil if the sun does not set.
moonrise (date, loc)	Moonrise on date at `loc` as a moment, or nil if the moon does not rise.
moonset (date, loc)	Moonset on date at `loc` as a moment, or nil if the moon does not set.

calendar (spec)

A calendar object: a built-in calendar by name, or a new calendar.

calendar(name) returns the built-in calendar name (e.g. "hebrew"), or nil if there is none; see calendars.
calendar{name = …, granularities = {…}, from_rd = f, to_rd = g} makes a new calendar. granularities is the ordered list of field names; from_rd maps an RD to a positional table of fields and to_rd (optional) maps a positional table back to an RD — the field names are matched to those positions. Omit to_rd for a read-only calendar, one that cannot be used to construct a date from fields. name is only for display (tostring); it is not registered, so a calendar of your own never clashes with a built-in one or another package’s.

A calendar object can be passed wherever a calendar name can. It is read-only and has the fields name and granularities.

Parameters:

spec string or table A built-in calendar name, or the definition of a new calendar.

Returns:

table or nil

Raises:

If a definition lacks granularities or from_rd.

Usage:

local hebrew = calendrica.calendar("hebrew")

local mine = calendrica.calendar{name = "mine", granularities = {"year", "day"},
   from_rd = function(rd) return {rd // 365, rd % 365} end}

date (source, calendar)

Express a date in a calendar (a built-in name or a calendar object).

The first argument is one of:

a field table — named fields interpreted in the given calendar, e.g. date({year=2024, month=1, day=1}, "gregorian");
an existing date (from date or today), re-expressed in the given calendar, e.g. date(d, "hebrew");
a fixed Rata Die integer, e.g. date(739758, "gregorian") (the inverse of .rd).

Parameters:

source table or number Field table, date object, or RD integer.
calendar string or table Calendar name (e.g. "gregorian"; see calendars) or calendar object.

Returns:

table

d.year

Raises:

If the calendar is unknown, if constructing from fields on a read-only calendar, or if source is not a date object, field table, or RD integer.

today ([calendar])

Return today’s date in the given calendar.

Parameters:

calendar string or table Calendar name or object. Defaults to "gregorian". (optional)

Returns:

table

Raises:

If the calendar name is unknown.

holiday (name, g_year[, calendar])

Return occurrences of a built-in holiday in a Gregorian year as date objects.

Parameters:

name string Holiday name (e.g. "easter", "hanukkah"). See holiday_names for the full list.
g_year number Gregorian year.
calendar string or table Calendar name or object for the returned dates. Defaults to "gregorian". (optional)

Returns:

{table,...}

Raises:

If name is not a built-in holiday, or calendar is unknown.

holidays (g_year[, calendar])

Return all built-in holidays occurring in a Gregorian year, sorted by date.

Parameters:

g_year number Gregorian year.
calendar string or table Calendar name or object for the returned dates. Defaults to "gregorian". (optional)

Returns:

{{name=string,dates={table,...}},...}

{name, dates}

Raises:

If calendar is unknown.

holiday_names ()

Return a sorted list of the built-in holiday names.

Returns:

{string,...}

calendars ()

Return a sorted list of the built-in calendar names. calendar(name) gives each one’s object, with its ordered field names in .granularities.

Returns:

{string,...}

Usage:

for _, name in ipairs(calendrica.calendars()) do
  print(name, table.concat(calendrica.calendar(name).granularities, ", "))
end

location (latitude, longitude, elevation, zone)

Construct a location for use with astronomical functions.

Parameters:

latitude number Degrees north (negative = south); any value, folded over the poles onto [-90, 90].
longitude number Degrees east (negative = west); any value, wrapped into [-180, 180).
elevation number Metres above sea level (negative below).
zone number
```
  UTC offset in hours (e.g. 2 for UTC+2, 5.5 for UTC+5:30). This is
```
a raw offset — it is used only to shift between universal and standard time, so a value beyond the usual civil range (-12 to 14) simply shifts the moment (past ±24 h, the day).

Returns:

table

.latitude

.longitude

.elevation

.zone

lunar_phase (date)

Lunar phase (0–360°) at midnight of date. 0 = new moon, 90 = first quarter, 180 = full moon, 270 = last quarter.

Parameters:

date table or number A date object or an RD integer.

Returns:

number

lunar_phases (phase, g_year[, calendar])

List of date objects on which the moon reaches lunar phase phase during Gregorian year g_year.

Note: this function’s name is provisional and likely will change in a future version.

Parameters:

phase number Lunar phase in degrees: 0 = new moon, 90 = first quarter, 180 = full moon, 270 = last quarter.
g_year number Gregorian year.
calendar string or table Calendar name or object for the returned dates. Defaults to "gregorian". (optional)

Returns:

{table,...}

Raises:

If calendar is unknown.

Usage:

local new_moons  = calendrica.lunar_phases(0,   2026)

local full_moons = calendrica.lunar_phases(180, 2026)
TODO: the name lunar_phases is provisional — it takes a phase and returns dates,
so it reads as a near-inverse of lunar_phase; revisit before a stable release.

sunrise (date, loc)

Sunrise on date at loc as a moment, or nil if the sun does not rise. The result is a date object (in date’s calendar, or Gregorian if date is an RD integer) carrying the time-of-day: .hour, .minute, .second, .zone, and the fractional .moment. Reproject it to another calendar with :to(name) and the time is preserved.

Parameters:

date table or number A date object or an RD integer.
loc table A location (from location).

Returns:

table or nil

sunset (date, loc)

Sunset on date at loc as a moment, or nil if the sun does not set. See sunrise for the returned moment’s fields.

Parameters:

date table or number A date object or an RD integer.
loc table A location (from location).

Returns:

table or nil

moonrise (date, loc)

Moonrise on date at loc as a moment, or nil if the moon does not rise. See sunrise for the returned moment’s fields.

Parameters:

date table or number A date object or an RD integer.
loc table A location (from location).

Returns:

table or nil

moonset (date, loc)

Moonset on date at loc as a moment, or nil if the moon does not set. See sunrise for the returned moment’s fields.

Parameters:

date table or number A date object or an RD integer.
loc table A location (from location).

Returns:

table or nil

lua-calendrica

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Module calendrica

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Module `calendrica`