diff --git a/homeassistant/components/calendar/__init__.py b/homeassistant/components/calendar/__init__.py
index 876b90eac9b..390e14d1689 100644
--- a/homeassistant/components/calendar/__init__.py
+++ b/homeassistant/components/calendar/__init__.py
@@ -386,6 +386,8 @@ class CalendarEventView(http.HomeAssistantView):
             return web.Response(status=HTTPStatus.BAD_REQUEST)
         if start_date is None or end_date is None:
             return web.Response(status=HTTPStatus.BAD_REQUEST)
+        if start_date > end_date:
+            return web.Response(status=HTTPStatus.BAD_REQUEST)
 
         try:
             calendar_event_list = await entity.async_get_events(
diff --git a/tests/components/calendar/test_init.py b/tests/components/calendar/test_init.py
index b5135ccc789..806410c9834 100644
--- a/tests/components/calendar/test_init.py
+++ b/tests/components/calendar/test_init.py
@@ -28,9 +28,7 @@ async def test_events_http_api(
     start = dt_util.now()
     end = start + timedelta(days=1)
     response = await client.get(
-        "/api/calendars/calendar.calendar_1?start={}&end={}".format(
-            start.isoformat(), end.isoformat()
-        )
+        f"/api/calendars/calendar.calendar_1?start={start.isoformat()}&end={end.isoformat()}"
     )
     assert response.status == HTTPStatus.OK
     events = await response.json()
@@ -63,14 +61,27 @@ async def test_events_http_api_error(
         side_effect=HomeAssistantError("Failure"),
     ):
         response = await client.get(
-            "/api/calendars/calendar.calendar_1?start={}&end={}".format(
-                start.isoformat(), end.isoformat()
-            )
+            f"/api/calendars/calendar.calendar_1?start={start.isoformat()}&end={end.isoformat()}"
         )
         assert response.status == HTTPStatus.INTERNAL_SERVER_ERROR
         assert await response.json() == {"message": "Error reading events: Failure"}
 
 
+async def test_events_http_api_dates_wrong_order(
+    hass: HomeAssistant, hass_client: ClientSessionGenerator
+) -> None:
+    """Test the calendar demo view."""
+    await async_setup_component(hass, "calendar", {"calendar": {"platform": "demo"}})
+    await hass.async_block_till_done()
+    client = await hass_client()
+    start = dt_util.now()
+    end = start + timedelta(days=-1)
+    response = await client.get(
+        f"/api/calendars/calendar.calendar_1?start={start.isoformat()}&end={end.isoformat()}"
+    )
+    assert response.status == HTTPStatus.BAD_REQUEST
+
+
 async def test_calendars_http_api(
     hass: HomeAssistant, hass_client: ClientSessionGenerator
 ) -> None: