# `Calendrical.Islamic.Rgsa` Implementation of the *Saudi Arabian sighting-based* Islamic calendar (CLDR `:islamic_rgsa`). Like `Calendrical.Islamic.Observational`, this calendar determines the start of each lunar month from actual crescent visibility, but the observation point is **Mecca, Saudi Arabia** (21.4225° N, 39.8262° E, 277 m) — the *al-Masjid al-Ḥarām* — rather than Cairo. This matches the location used for the canonical Saudi religious determination of dates such as the start of Ramadan and Eid. This calendar differs from `Calendrical.Islamic.UmmAlQura` in two important ways: * **`UmmAlQura`** uses the official tabular calendar published by KACST (the Umm al-Qura Calendar). It is a precomputed lookup table — fast, deterministic, and the legal Saudi civil calendar. * **`Rgsa`** computes month starts from astronomical visibility at Mecca on demand. It is the algorithmic equivalent of the Saudi *religious sighting* practice and may diverge from `UmmAlQura` in months where the astronomical prediction does not match the KACST table. Days are assumed to begin at midnight. ## Visibility model As with `Calendrical.Islamic.Observational`, crescent visibility is computed by `Astro.new_visible_crescent/3` using the Odeh (2006) criterion by default. ## Reference - CLDR `:islamic_rgsa` calendar type. The `rgsa` suffix denotes "*Religious Saudi Arabia*". - Dershowitz & Reingold, *Calendrical Calculations* (4th ed.), Chapter 14. # `day` ```elixir @type day() :: 1..30 ``` # `month` ```elixir @type month() :: 1..12 ``` # `year` ```elixir @type year() :: integer() ``` # `calendar_base` Identifies whether this calendar is month or week based. # `calendar_year` ```elixir @spec calendar_year(Calendar.year(), Calendar.month(), Calendar.day()) :: Calendar.year() ``` Returns the calendar year as displayed on rendered calendars. # `cldr_calendar_type` Defines the CLDR calendar type for this calendar. This type is used in support of `Calendrical. localize/3`. # `cyclic_year` ```elixir @spec cyclic_year(Calendar.year(), Calendar.month(), Calendar.day()) :: Calendar.year() ``` Returns the cyclic year as displayed on rendered calendars. # `date_from_iso_days` ```elixir @spec date_from_iso_days(integer()) :: {year(), month(), day()} ``` Returns a Saudi sighting-based Islamic `{year, month, day}` for the given ISO day number. ### Arguments * `iso_days` is an integer count of days since the proleptic ISO epoch. ### Returns * A three-tuple `{year, month, day}` in the Saudi sighting-based Islamic calendar. * Raises `Calendrical.UnsupportedDateRangeError` if `iso_days` falls outside the range of the underlying JPL ephemeris. ### Examples iex> Calendrical.Islamic.Rgsa.date_from_iso_days(739_252) {1445, 6, 20} # `date_to_iso_days` ```elixir @spec date_to_iso_days(year(), month(), day()) :: integer() ``` Returns the number of ISO days for the given Saudi sighting-based Islamic `year`, `month`, and `day`. ### Arguments * `year` is any Hijri year as an integer. * `month` is a Hijri month in the range `1..12`. * `day` is a Hijri day-of-month in the range `1..30`. ### Returns * An integer count of days since the proleptic ISO epoch. * Raises `Calendrical.UnsupportedDateRangeError` if the date falls outside the range of the underlying JPL ephemeris. ### Examples iex> Calendrical.Islamic.Rgsa.date_to_iso_days(1446, 1, 1) 739439 # `day_of_era` ```elixir @spec day_of_era(Calendar.year(), Calendar.month(), Calendar.day()) :: {day :: Calendar.day(), era :: Calendar.era()} ``` Calculates the day and era from the given `year`, `month`, and `day`. By default we consider on two eras: before the epoch and on-or-after the epoch. # `day_of_year` ```elixir @spec day_of_year(Calendar.year(), Calendar.month(), Calendar.day()) :: Calendar.day() ``` Calculates the day of the year from the given `year`, `month`, and `day`. # `days_in_month` ```elixir @spec days_in_month(Calendar.month()) :: Calendar.month() | {:ambiguous, Range.t() | [pos_integer()]} | {:error, :undefined} ``` Returns how many days there are in the given month. Must be implemented in derived calendars because we cannot know what the calendar format is. # `days_in_month` ```elixir @spec days_in_month(Calendar.year(), Calendar.month()) :: Calendar.month() @spec days_in_month(year(), month()) :: 29..30 ``` Returns the number of days in the given Hijri `year` and `month` (29 or 30, determined by predicted crescent visibility at Mecca). ### Arguments * `year` is any Hijri year as an integer. * `month` is a Hijri month in the range `1..12`. ### Returns * The number of days in the month (`29` or `30`). ### Examples iex> Calendrical.Islamic.Rgsa.days_in_month(1446, 1) 30 iex> Calendrical.Islamic.Rgsa.days_in_month(1446, 4) 29 # `days_in_week` Returns the number days in a a week. # `days_in_year` ```elixir @spec days_in_year(year()) :: 354..355 ``` Returns the number of days in the given Hijri `year`. ### Arguments * `year` is any Hijri year as an integer. ### Returns * `354` for an ordinary year or `355` for a leap year. ### Examples iex> Calendrical.Islamic.Rgsa.days_in_year(1446) 354 # `epoch` # `epoch_day_of_week` # `extended_year` ```elixir @spec extended_year(Calendar.year(), Calendar.month(), Calendar.day()) :: Calendar.year() ``` Returns the extended year as displayed on rendered calendars. # `first_day_of_week` # `iso_week_of_year` ```elixir @spec iso_week_of_year(Calendar.year(), Calendar.month(), Calendar.day()) :: {:error, :not_defined} ``` Calculates the ISO week of the year from the given `year`, `month`, and `day`. By default this function always returns `{:error, :not_defined}`. # `last_day_of_week` # `leap_year?` ```elixir @spec leap_year?(year()) :: boolean() ``` Returns whether the given Hijri `year` is a 355-day year (the sighting-based analogue of a "leap year"). Year length varies between 354 and 355 days depending on predicted crescent sightings at Mecca, so this is computed by comparing the starts of two successive years. ### Arguments * `year` is any Hijri year as an integer. ### Returns * `true` if the year contains 355 days; otherwise `false`. ### Examples iex> Calendrical.Islamic.Rgsa.leap_year?(1447) true iex> Calendrical.Islamic.Rgsa.leap_year?(1446) false # `location` ```elixir @spec location() :: Geo.PointZ.t() ``` Returns the geographic location used to determine crescent visibility for this calendar. ### Returns * A `t:Geo.PointZ.t/0` for Mecca, Saudi Arabia (the *al-Masjid al-Ḥarām*). ### Examples iex> Calendrical.Islamic.Rgsa.location().coordinates {39.8262, 21.4225, 277.0} # `month` Returns a `t:Date.Range.t/0` representing a given month of a year. # `month_of_year` ```elixir @spec month_of_year(Calendar.year(), Calendar.month(), Calendar.day()) :: Calendar.month() | {Calendar.month(), Calendrical.leap_month?()} ``` Returns the month of the year from the given `year`, `month`, and `day`. # `months_in_leap_year` Returns the number of months in a leap year. # `months_in_ordinary_year` Returns the number of months in a normal year. # `months_in_year` Returns the number of months in a year, without a year. Returns an integer when every year has the same number of months, or `{:ambiguous, first..last}` for lunisolar calendars whose year length varies (e.g. the Hebrew calendar's 12 or 13 months). # `months_in_year` Returns the number of months in a given `year`. # `naive_datetime_from_iso_days` ```elixir @spec naive_datetime_from_iso_days(Calendar.iso_days()) :: {Calendar.year(), Calendar.month(), Calendar.day(), Calendar.hour(), Calendar.minute(), Calendar.second(), Calendar.microsecond()} ``` Converts the `t:Calendar.iso_days` format to the datetime format specified by this calendar. # `naive_datetime_to_iso_days` ```elixir @spec naive_datetime_to_iso_days( Calendar.year(), Calendar.month(), Calendar.day(), Calendar.hour(), Calendar.minute(), Calendar.second(), Calendar.microsecond() ) :: Calendar.iso_days() ``` Returns the `t:Calendar.iso_days` format of the specified date. # `periods_in_year` Returns the number of periods in a given `year`. A period corresponds to a month in month-based calendars and a week in week-based calendars. # `plus` Adds an `increment` number of `date_part`s to a `year-month-day`. `date_part` can be `:years`, `:months`, `:weeks` or `:days`. # `quarter` Returns a `t:Date.Range.t/0` representing a given quarter of a year. # `quarter_of_year` ```elixir @spec quarter_of_year(Calendar.year(), Calendar.month(), Calendar.day()) :: Calendrical.quarter() ``` Returns the quarter of the year from the given `year`, `month`, and `day`. # `related_gregorian_year` ```elixir @spec related_gregorian_year(Calendar.year(), Calendar.month(), Calendar.day()) :: Calendar.year() ``` Returns the related Gregorian year as displayed on rendered calendars. Per TR35 this is the Gregorian year in which this calendar's `year` begins, so it is constant for every date of the calendar year. # `valid_date?` ```elixir @spec valid_date?(year(), month(), day()) :: boolean() ``` Returns whether the given `year`, `month`, and `day` form a valid Saudi sighting-based Islamic date. ### Arguments * `year` is any positive Hijri year as an integer. * `month` is a Hijri month in the range `1..12`. * `day` is a Hijri day-of-month in the range `1..30`. ### Returns * `true` if the date is valid; otherwise `false`. ### Examples iex> Calendrical.Islamic.Rgsa.valid_date?(1446, 1, 30) true iex> Calendrical.Islamic.Rgsa.valid_date?(1446, 4, 30) false # `week` Returns a `t:Date.Range.t/0` representing a given week of a year. # `week_of_month` ```elixir @spec week_of_month(Calendar.year(), Calendar.month(), Calendar.day()) :: {pos_integer(), pos_integer()} | {:error, :not_defined} ``` Calculates the week of the year from the given `year`, `month`, and `day`. By default this function always returns `{:error, :not_defined}`. # `week_of_year` ```elixir @spec week_of_year(Calendar.year(), Calendar.month(), Calendar.day()) :: {:error, :not_defined} ``` Calculates the week of the year from the given `year`, `month`, and `day`. By default this function always returns `{:error, :not_defined}`. # `weeks_in_year` Returns the number of weeks in a given `year`. # `year` Returns a `t:Date.Range.t/0` representing a given year. # `year_of_era` ```elixir @spec year_of_era(Calendar.year()) :: {year :: Calendar.year(), era :: Calendar.era()} ``` Calculates the year and era from the given `year`. # `year_of_era` ```elixir @spec year_of_era(Calendar.year(), Calendar.month(), Calendar.day()) :: {year :: Calendar.year(), era :: Calendar.era()} ``` Calculates the year and era from the given `date`. --- *Consult [api-reference.md](api-reference.md) for complete listing*