Fixed #36679 -- Fixed Basque date formats to use parenthetical declension suffixes.

Basque (eu) grammar requires conditional suffixes on years and day
articles that depend on the final sound of the preceding word. Since
Django's format strings are static, the CLDR parenthetical convention
("(e)ko" instead of "ko") is used to express the optionality.

Co-authored-by: Natalia <124304+nessita@users.noreply.github.com>

2 files changed, 23 insertions, 3 deletions

diff --git a/django/conf/locale/eu/formats.py b/django/conf/locale/eu/formats.py
index 61b16fbc6f..e707f931c7 100644
--- a/django/conf/locale/eu/formats.py
+++ b/django/conf/locale/eu/formats.py
@@ -2,10 +2,10 @@
 #
 # The *_FORMAT strings use the Django date format syntax,
 # see https://docs.djangoproject.com/en/dev/ref/templates/builtins/#date
-DATE_FORMAT = r"Y\k\o N j\a"
+DATE_FORMAT = r"Y(\e)\k\o N\k j"
 TIME_FORMAT = "H:i"
-DATETIME_FORMAT = r"Y\k\o N j\a, H:i"
-YEAR_MONTH_FORMAT = r"Y\k\o F"
+DATETIME_FORMAT = r"Y(\e)\k\o N\k j, H:i"
+YEAR_MONTH_FORMAT = r"Y(\e)\k\o F"
 MONTH_DAY_FORMAT = r"F\r\e\n j\a"
 SHORT_DATE_FORMAT = "Y-m-d"
 SHORT_DATETIME_FORMAT = "Y-m-d H:i"
diff --git a/tests/i18n/tests.py b/tests/i18n/tests.py
index e593d97cba..91e7c95f67 100644
--- a/tests/i18n/tests.py
+++ b/tests/i18n/tests.py
@@ -1179,6 +1179,26 @@ class FormattingTests(SimpleTestCase):
                 with translation.override(locale, deactivate=True):
                     self.assertEqual(expected, format_function(*format_args))
 
+    def test_basque_date_formats(self):
+        # Basque locale uses parenthetical suffixes for conditional declension:
+        # (e)ko for years and declined month/day forms.
+        with translation.override("eu", deactivate=True):
+            self.assertEqual(date_format(self.d), "2009(e)ko abe.k 31")
+            self.assertEqual(
+                date_format(self.dt, "DATETIME_FORMAT"),
+                "2009(e)ko abe.k 31, 20:50",
+            )
+            self.assertEqual(
+                date_format(self.d, "YEAR_MONTH_FORMAT"), "2009(e)ko abendua"
+            )
+            self.assertEqual(date_format(self.d, "MONTH_DAY_FORMAT"), "abenduaren 31a")
+            # Day 11 (hamaika in Basque) ends in 'a' as a word, but the
+            # numeral form does not, so appending 'a' is correct here.
+            self.assertEqual(
+                date_format(datetime.date(2009, 12, 11), "MONTH_DAY_FORMAT"),
+                "abenduaren 11a",
+            )
+
     def test_sub_locales(self):
         """
         Check if sublocales fall back to the main locale