validator: Reject ISO 8601 dates missing leading zeros.

Signed-off-by: Anders Kaseorg <anders@zulip.com>

zerver/migrations/0306_custom_profile_field_date_format.py

@@ -0,0 +1,27 @@
+from django.db import migrations
+
+
+class Migration(migrations.Migration):
+    """
+    We previously accepted invalid ISO 8601 dates like 1909-3-5 for
+    date values of custom profile fields. Correct them by adding the
+    missing leading zeros: 1909-03-05.
+    """
+
+    dependencies = [
+        ("zerver", "0305_realm_deactivated_redirect"),
+    ]
+
+    operations = [
+        migrations.RunSQL(
+            sql="""\
+                UPDATE zerver_customprofilefieldvalue
+                SET value = to_char(to_date(value, 'YYYY-MM-DD'), 'YYYY-MM-DD')
+                FROM zerver_customprofilefield AS f
+                WHERE f.id = field_id
+                AND f.field_type = 4
+                AND value <> to_char(to_date(value, 'YYYY-MM-DD'), 'YYYY-MM-DD');
+            """,
+            reverse_sql="",
+        ),
+    ]