Fix days in month calculation for the last month

Author: kipcole9

CHANGELOG.md

@@ -2,6 +2,18 @@
 
 **Note that `ex_cldr_calendars` version 1.24.0 and later are supported on Elixir 1.12 and later only.**
 
+## Cldr.Calendars v2.4.0
+
+This is the changelog for Cldr Calendars v2.4.0 released on _____, 2025.  For older changelogs please consult the release tag on [GitHub](https://github.com/elixir-cldr/cldr_calendars/tags)
+
+### Bug Fixes
+
+### Enhancements
+
+* Adds `use Cldr.Calendar.Julian, new_year_starting_month_and_day: {month_of_year, day_of_month}` to allow modelling Julian calendars that don't start on January 1st.
+
+* Adds `Cldr.Calendar.convert/2` which converts either a date or a date range to another calendar.
+
 ## Cldr.Calendars v2.3.1
 
 This is the changelog for Cldr Calendars v2.3.1 released on September 10th, 2025.  For older changelogs please consult the release tag on [GitHub](https://github.com/elixir-cldr/cldr_calendars/tags)

lib/cldr/calendar.ex

@@ -798,6 +798,46 @@ defmodule Cldr.Calendar do
     end
   end
 
+  @doc """
+  Converts a date or a date range to another calendar.
+
+  If the argument is a `t:Date.t/0`, `Date.convert/2` is used to
+  do the conversion.
+
+  If the argument is a `t:/Date.Range.t/0` then `Date.convert/2` is
+  applied to the start and end of the range and a new range returned.
+
+  ### Arguments
+
+  * `date_or_range` is any `t:Date.t/0` or ` t:Date.Range.t/0`.
+
+  * `calendar` is any valid calendar module.
+
+  ### Returns
+
+  * `{:ok, converted_date_or_range}` or
+
+  * `{:error, :incompatible_calendars}`
+
+  ### Example
+
+  """
+  @doc since: "2.4.0"
+  @spec convert(Date.t() | Date.Range.t(), Calendar.calendar()) ::
+    {:ok, Date.t() | Date.Range.t()} | {:error, :incompatible_calendars}
+
+  def convert(%Date{} = date, calendar) do
+    Date.convert(date, calendar)
+  end
+
+  def convert(%Date.Range{first: first, last: last, step: step}, calendar) do
+    with {:ok, new_first} <- Date.convert(first, calendar),
+         {:ok, new_last} <- Date.convert(last, calendar) do
+      {:ok, Date.range(new_first, new_last, step)}
+    end
+  end
+
+
   @doc """
   Formats the given date, time, or datetime into a string.
 

lib/cldr/calendar/calendars/julian.ex

@@ -18,6 +18,9 @@ defmodule Cldr.Calendar.Julian do
       @new_year_starting_month start_month
       @new_year_starting_day start_day
 
+      @months_in_year Cldr.Calendar.Julian.months_in_year(0)
+      @last_month_of_year rem(start_month + (@months_in_year - 1), @months_in_year)
+
       # Adjust the year to be a Jan 1st starting year and carry
       # on
       def date_to_iso_days(year, month, day) when month <= @new_year_starting_month and day < @new_year_starting_day do
@@ -28,6 +31,10 @@ defmodule Cldr.Calendar.Julian do
         Cldr.Calendar.Julian.date_to_iso_days(year, month, day)
       end
 
+      def naive_datetime_to_iso_days(year, month, day, 0, 0, 0, {0, 0}) do
+        {date_to_iso_days(year, month, day), {0, 0}}
+      end
+
       # Adjust the year to be this calendars starting year
       def date_from_iso_days(iso_days) do
         {year, month, day} = Cldr.Calendar.Julian.date_from_iso_days(iso_days)
@@ -39,22 +46,78 @@ defmodule Cldr.Calendar.Julian do
         end
       end
 
+      # Here we use month to mean ordinal month. Therefore if the calendar
+      # starts on March 25th, then days in month for March will be seen as
+      # days if month for month 1.
+
+      def days_in_month(year, ordinal_month) do
+        adjusted_month =
+          Cldr.Math.amod(ordinal_month + @new_year_starting_month - 1, @months_in_year)
+
+        adjusted_year =
+          if adjusted_month < @new_year_starting_month, do: year + 1, else: year
+
+        cond do
+          # The first month of the year will be short since the year starts
+          # part way through the month
+          adjusted_month == @new_year_starting_month ->
+            days_in_julian_month = Cldr.Calendar.Julian.days_in_month(year, ordinal_month)
+            days_in_julian_month - @new_year_starting_day
+
+          # The last month of the year will be "long" since the first part of the
+          # first month that is before the start of year will be included
+          adjusted_month == @last_month_of_year ->
+            start_of_month =
+              date_to_iso_days(adjusted_year, adjusted_month, 1)
+            start_of_next_month =
+              date_to_iso_days(adjusted_year, @new_year_starting_month, @new_year_starting_day)
+            start_of_next_month - start_of_month
+
+          true ->
+            Cldr.Calendar.Julian.days_in_month(year, adjusted_month)
+        end
+      end
+
+      def month(year, ordinal_month) do
+        adjusted_month =
+          Cldr.Math.amod(ordinal_month + @new_year_starting_month - 1, @months_in_year)
+
+        adjusted_year =
+          if adjusted_month < @new_year_starting_month, do: year + 1, else: year
+
+        first_day =
+          if adjusted_month == @new_year_starting_month, do: @new_year_starting_day, else: 1
+
+        {:ok, first} = Date.new(adjusted_year, adjusted_month, first_day, __MODULE__)
+        first_iso_days = date_to_iso_days(year, adjusted_month, first_day)
+        days_in_month = days_in_month(year, ordinal_month)
+
+        last_iso_days = first_iso_days + days_in_month - 1
+        {year, month, day} = date_from_iso_days(last_iso_days)
+        {:ok, last} = Date.new(year, month, day, __MODULE__)
+
+        Date.range(first, last, 1)
+      end
+
+      # Returns the ordinal month
+      def month_of_year(_year, ordinal_month, _day) do
+        Cldr.Math.amod(ordinal_month - @new_year_starting_month + 1, @months_in_year)
+      end
+
       defdelegate valid_date?(year, month, day), to: Cldr.Calendar.Julian
       defdelegate leap_year?(year), to: Cldr.Calendar.Julian
       defdelegate plus(year, month, day, part, years, options), to: Cldr.Calendar.Julian
       defdelegate week(year, week), to: Cldr.Calendar.Julian
-      defdelegate month(year, month), to: Cldr.Calendar.Julian
       defdelegate weeks_in_year(year), to: Cldr.Calendar.Julian
-      defdelegate days_in_month(year, month), to: Cldr.Calendar.Julian
       defdelegate day_of_week(year, month, day, starts_on), to: Cldr.Calendar.Julian
       defdelegate day_of_year(year, month, day), to: Cldr.Calendar.Julian
       defdelegate day_of_era(year, month, day), to: Cldr.Calendar.Julian
       defdelegate iso_week_of_year(year, month, day), to: Cldr.Calendar.Julian
       defdelegate week_of_year(year, month, day), to: Cldr.Calendar.Julian
-      defdelegate month_of_year(year, month, day), to: Cldr.Calendar.Julian
       defdelegate quarter_of_year(year, month, day), to: Cldr.Calendar.Julian
       defdelegate year_of_era(year), to: Cldr.Calendar.Julian
       defdelegate parse_date(date), to: Cldr.Calendar.Julian
+      defdelegate date_to_string(year, month, day), to: Cldr.Calendar.Julian
     end
   end
 

mix.exs

@@ -1,7 +1,7 @@
 defmodule Cldr.Calendar.MixProject do
   use Mix.Project
 
-  @version "2.3.1"
+  @version "2.4.0"
 
   def project do
     [