diff --git a/src/demetriek/device.py b/src/demetriek/device.py
index 7b802b66..937b7144 100644
--- a/src/demetriek/device.py
+++ b/src/demetriek/device.py
@@ -13,6 +13,7 @@
 from aiohttp.helpers import BasicAuth
 from yarl import URL
 
+from .const import ScreensaverMode
 from .exceptions import (
     LaMetricAuthenticationError,
     LaMetricConnectionError,
@@ -32,7 +33,7 @@
 if TYPE_CHECKING:
     from datetime import time
 
-    from .const import BrightnessMode, DeviceMode, ScreensaverMode
+    from .const import BrightnessMode, DeviceMode
 
 
 @dataclass
@@ -203,7 +204,25 @@ async def display(  # noqa: PLR0913 # pylint: disable=too-many-arguments
             A Display object, with latest or updated information about
             the display of the LaMetric device.
 
+        Raises:
+        ------
+            ValueError: The time based mode was given without both of
+                its times.
+
         """
+        # The device wants both times on every time based write, even one
+        # that only toggles the mode. It rejects a lone end time and
+        # quietly ignores a lone start time, so catch that here rather
+        # than let it look like it worked.
+        if screensaver_mode is ScreensaverMode.TIME_BASED and (
+            screensaver_start_time is None or screensaver_end_time is None
+        ):
+            msg = (
+                "The time based screensaver mode needs both"
+                " screensaver_start_time and screensaver_end_time"
+            )
+            raise ValueError(msg)
+
         data: dict[str, Any] = {}
 
         if brightness is not None:
diff --git a/tests/test_display.py b/tests/test_display.py
index 8e233eea..0e51ada7 100644
--- a/tests/test_display.py
+++ b/tests/test_display.py
@@ -4,6 +4,7 @@
 from datetime import time
 
 import aiohttp
+import pytest
 from aresponses import Response, ResponsesMockServer
 
 from demetriek import LaMetricDevice
@@ -161,3 +162,52 @@ async def response_handler(request: aiohttp.ClientResponse) -> Response:
     async with aiohttp.ClientSession() as session:
         demetriek = LaMetricDevice(host="127.0.0.2", api_key="abc", session=session)
         await demetriek.display(screensaver_mode=ScreensaverMode.WHEN_DARK)
+
+
+@pytest.mark.parametrize(
+    ("start_time", "end_time"),
+    [
+        (time(23, 0, 0), None),
+        (None, time(7, 0, 0)),
+        (None, None),
+    ],
+)
+async def test_set_display_screensaver_time_based_needs_both_times(
+    start_time: time | None,
+    end_time: time | None,
+) -> None:
+    """Test the time based mode is rejected without both of its times."""
+    demetriek = LaMetricDevice(host="127.0.0.2", api_key="abc")
+    with pytest.raises(ValueError, match="needs both"):
+        await demetriek.display(
+            screensaver_mode=ScreensaverMode.TIME_BASED,
+            screensaver_mode_enabled=True,
+            screensaver_start_time=start_time,
+            screensaver_end_time=end_time,
+        )
+
+
+async def test_set_display_screensaver_when_dark_needs_no_times(
+    aresponses: ResponsesMockServer,
+) -> None:
+    """Test the other modes are unaffected by the time based requirement."""
+
+    async def response_handler(request: aiohttp.ClientResponse) -> Response:
+        """Response handler for this test."""
+        assert await request.json() == {
+            "screensaver": {"mode": "when_dark", "mode_params": {"enabled": True}},
+        }
+        return aresponses.Response(
+            status=200,
+            headers={"Content-Type": "application/json"},
+            text=load_fixture("display_set_screensaver.json"),
+        )
+
+    aresponses.add("127.0.0.2:4343", "/api/v2/device/display", "PUT", response_handler)
+
+    async with aiohttp.ClientSession() as session:
+        demetriek = LaMetricDevice(host="127.0.0.2", api_key="abc", session=session)
+        await demetriek.display(
+            screensaver_mode=ScreensaverMode.WHEN_DARK,
+            screensaver_mode_enabled=True,
+        )