fix(export): emit date custom-field values as ISO-8601 for re-import

lib/mv/membership/custom_field_value_formatter.ex

@@ -4,13 +4,13 @@ defmodule Mv.Membership.CustomFieldValueFormatter do
 
   Same logic as the member overview Formatter but without Gettext or web helpers,
   so it can be used from the Membership context. For boolean: "Yes"/"No";
-  for date: European format (dd.mm.yyyy).
+  for date: ISO-8601 (YYYY-MM-DD) so exported values can be re-imported.
   """
   @doc """
   Formats a custom field value for plain text (e.g. CSV).
 
   Handles nil, Ash.Union, JSONB map, and direct values. Uses custom_field.value_type
-  for typing. Boolean -> "Yes"/"No", Date -> dd.mm.yyyy.
+  for typing. Boolean -> "Yes"/"No", Date -> ISO-8601 (YYYY-MM-DD).
   """
   def format_custom_field_value(nil, _custom_field), do: ""
 
@@ -18,6 +18,10 @@ defmodule Mv.Membership.CustomFieldValueFormatter do
     format_value_by_type(value, type, custom_field)
   end
 
+  def format_custom_field_value(%Date{} = value, custom_field) do
+    format_value_by_type(value, :date, custom_field)
+  end
+
   def format_custom_field_value(value, custom_field) when is_map(value) do
     type = Map.get(value, "type") || Map.get(value, "_union_type")
     val = Map.get(value, "value") || Map.get(value, "_union_value")
@@ -41,12 +45,12 @@ defmodule Mv.Membership.CustomFieldValueFormatter do
   defp format_value_by_type(value, :boolean, _), do: to_string(value)
 
   defp format_value_by_type(%Date{} = date, :date, _) do
-    Calendar.strftime(date, "%d.%m.%Y")
+    Date.to_iso8601(date)
   end
 
   defp format_value_by_type(value, :date, _) when is_binary(value) do
     case Date.from_iso8601(value) do
-      {:ok, date} -> Calendar.strftime(date, "%d.%m.%Y")
+      {:ok, date} -> Date.to_iso8601(date)
       _ -> value
     end
   end

test/mv/membership/custom_field_value_formatter_test.exs

@@ -0,0 +1,27 @@
+defmodule Mv.Membership.CustomFieldValueFormatterTest do
+  use ExUnit.Case, async: true
+
+  alias Mv.Membership.CustomFieldValueFormatter
+
+  describe "format_custom_field_value/2 for :date" do
+    test "formats an Ash.Union date value as ISO-8601" do
+      union = %Ash.Union{value: ~D[2024-03-15], type: :date}
+
+      assert CustomFieldValueFormatter.format_custom_field_value(union, %{value_type: :date}) ==
+               "2024-03-15"
+    end
+
+    test "formats a direct Date value as ISO-8601" do
+      assert CustomFieldValueFormatter.format_custom_field_value(~D[2024-03-15], %{
+               value_type: :date
+             }) == "2024-03-15"
+    end
+
+    test "formats an already-stored ISO-8601 string date as ISO-8601" do
+      union = %Ash.Union{value: "2024-03-15", type: :date}
+
+      assert CustomFieldValueFormatter.format_custom_field_value(union, %{value_type: :date}) ==
+               "2024-03-15"
+    end
+  end
+end