# `Calendrical.Islamic.Civil`

Implementation of the tabular Islamic (Hijri) civil calendar.

The civil tabular calendar is a 12-month lunar calendar with a fixed
354- or 355-day year computed from a 30-year cycle in which years
2, 5, 7, 10, 13, 16, 18, 21, 24, 26 and 29 are leap years (the
Type II *Kūshyār* cycle).

The civil epoch is **Friday 16 July 622 CE (Julian)** — equivalent
to **19 July 622 CE (proleptic Gregorian)** — the day after the
*hijra* of Muhammad from Mecca to Medina. This is the convention
used by most algorithmic calendars and CLDR's `islamic-civil`
calendar type.

Days are assumed to begin at midnight rather than at sunset.

## Month structure

| # | Name             | Days |
|---|------------------|------|
| 1 | Muharram         | 30   |
| 2 | Ṣafar            | 29   |
| 3 | Rabī' al-Awwal   | 30   |
| 4 | Rabī' al-Thānī   | 29   |
| 5 | Jumādā al-Ūlā    | 30   |
| 6 | Jumādā al-Ākhirah| 29   |
| 7 | Rajab            | 30   |
| 8 | Sha'bān          | 29   |
| 9 | Ramaḍān          | 30   |
| 10| Shawwāl          | 29   |
| 11| Dhū al-Qa'dah    | 30   |
| 12| Dhū al-Ḥijjah    | 29 / 30 (leap) |

# `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 civil Islamic `{year, month, day}` for the given ISO day
number.

# `date_to_iso_days`

```elixir
@spec date_to_iso_days(year(), month(), day()) :: integer()
```

Returns the number of ISO days for the given civil Islamic
`year`, `month`, and `day`.

# `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`.

# `days_in_week`

Returns the number days in a a week.

# `days_in_year`

Returns the number of days in the given Hijri `year` (354 or 355).

# `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 leap year.

# `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
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 `:months` only.

# `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 gregorain year as displayed
on rendered calendars.

# `valid_date?`

Determines if the `date` given is valid according to
this calendar.

# `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*