diff --git a/src/uds/REST/methods/calendarrules.py b/src/uds/REST/methods/calendarrules.py
index 4dfa08e72..47222c50b 100644
--- a/src/uds/REST/methods/calendarrules.py
+++ b/src/uds/REST/methods/calendarrules.py
@@ -81,8 +81,19 @@ class CalendarRuleItem(types.rest.BaseRestItem):
     id: str
     name: str
     comments: str
-    start: datetime.datetime
-    end: datetime.datetime | None
+    start: datetime.datetime = dataclasses.field(
+        metadata={
+            "description": "ISO 8601 date-time. Without UTC offset it is taken as UTC. "
+            "Integer timestamps are rejected."
+        }
+    )
+    end: datetime.datetime | None = dataclasses.field(
+        metadata={
+            "description": "Last day of the rule, ISO 8601 (date or date-time). Only the calendar day "
+            "is used, as sent, without converting it to UTC. Returned as the last instant of that day. "
+            "Null means no end."
+        }
+    )
     frequency: str
     interval: int
     duration: int
diff --git a/tests/REST/methods/calendars/__init__.py b/tests/REST/methods/calendars/__init__.py
new file mode 100644
index 000000000..e69de29bb
diff --git a/tests/REST/methods/calendars/test_rule_api_schema.py b/tests/REST/methods/calendars/test_rule_api_schema.py
new file mode 100644
index 000000000..ace6b5588
--- /dev/null
+++ b/tests/REST/methods/calendars/test_rule_api_schema.py
@@ -0,0 +1,50 @@
+#
+# Copyright (c) 2026 Virtual Cable S.L.
+# All rights reserved.
+#
+# Redistribution and use in source and binary forms, with or without modification,
+# are permitted provided that the following conditions are met:
+#
+#    * Redistributions of source code must retain the above copyright notice,
+#      this list of conditions and the following disclaimer.
+#    * Redistributions in binary form must reproduce the above copyright notice,
+#      this list of conditions and the following disclaimer in the documentation
+#      and/or other materials provided with the distribution.
+#    * Neither the name of Virtual Cable S.L. nor the names of its contributors
+#      may be used to endorse or promote products derived from this software
+#      without specific prior written permission.
+#
+# THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
+# AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+# IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
+# DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE
+# FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
+# DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
+# SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
+# CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
+# OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+# OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+"""
+Author: Andres Schumann, aschumann at virtualcable dot es
+"""
+
+import typing
+
+from tests.utils.test import UDSTestCase
+from uds.core import types
+from uds.REST.methods.calendarrules import CalendarRuleItem
+
+
+class CalendarRuleApiSchemaTest(UDSTestCase):
+    def _description(self, field: str) -> str:
+        schema = CalendarRuleItem.api_components().schemas["CalendarRuleItem"]
+        assert isinstance(schema, types.rest.api.Schema)
+        return typing.cast(str, schema.properties[field].description or "")
+
+    def test_start_documents_iso_8601(self) -> None:
+        self.assertIn("ISO 8601", self._description("start"))
+
+    def test_end_documents_iso_8601_calendar_day(self) -> None:
+        description = self._description("end")
+        self.assertIn("ISO 8601", description)
+        self.assertIn("calendar day", description)