diff --git a/RELEASE_NOTES.md b/RELEASE_NOTES.md
index 72e44a3..9f0393b 100644
--- a/RELEASE_NOTES.md
+++ b/RELEASE_NOTES.md
@@ -2,7 +2,7 @@
 
 ## Summary
 
-<!-- Here goes a general summary of what this release is about -->
+Raise `ValueError` when `start_time` passed to `create()` is neither a `datetime` nor `"NOW"`.
 
 ## Upgrading
 
@@ -14,4 +14,4 @@
 
 ## Bug Fixes
 
-<!-- Here goes notable bug fixes that are worth a special mention or explanation -->
+- `DispatchApiClient.create()`: Passing an invalid `start_time` (not a `datetime` or `"NOW"`) previously silently created a dispatch with an epoch timestamp (1970-01-01). It now raises `ValueError` immediately.
diff --git a/src/frequenz/client/dispatch/_client.py b/src/frequenz/client/dispatch/_client.py
index b09ba35..9381c4e 100644
--- a/src/frequenz/client/dispatch/_client.py
+++ b/src/frequenz/client/dispatch/_client.py
@@ -2,6 +2,7 @@
 # Copyright © 2024 Frequenz Energy-as-a-Service GmbH
 
 """Dispatch API client for Python."""
+
 from __future__ import annotations
 
 import warnings
@@ -344,6 +345,8 @@ async def create(  # pylint: disable=too-many-positional-arguments
                 or start_time.tzinfo.utcoffset(start_time) is None
             ):
                 raise ValueError("start_time must be timezone aware")
+        elif start_time != "NOW":
+            raise ValueError("start_time must be a datetime or 'NOW'")
 
         request = DispatchCreateRequest(
             microgrid_id=microgrid_id,
diff --git a/tests/test_client.py b/tests/test_client.py
index ca4289b..62153b2 100644
--- a/tests/test_client.py
+++ b/tests/test_client.py
@@ -6,7 +6,7 @@
 import asyncio
 import random
 from dataclasses import replace
-from datetime import timedelta
+from datetime import datetime, timedelta, timezone
 from functools import partial
 
 import grpc
@@ -471,3 +471,36 @@ async def expect(dispatch: Dispatch, event: Event) -> None:
 
     # Expect the first dispatch deletion
     await expect(dispatches[0], Event.DELETED)
+
+
+async def test_create_invalid_start_time(client: FakeClient, sample: Dispatch) -> None:
+    """Test that create() raises ValueError for invalid start_time types."""
+    microgrid_id = MicrogridId(random.randint(1, 100))
+    params = to_create_params(microgrid_id, sample)
+
+    for invalid in [0, 1234567890, "2026-01-01T00:00:00", object()]:
+        params["start_time"] = invalid
+        with raises(ValueError):
+            await client.create(**params)
+
+
+async def test_create_naive_datetime_raises(
+    client: FakeClient, sample: Dispatch
+) -> None:
+    """Test that create() raises ValueError for a timezone-naive datetime."""
+    microgrid_id = MicrogridId(random.randint(1, 100))
+    params = to_create_params(microgrid_id, sample)
+    params["start_time"] = datetime(2099, 1, 1, 0, 0, 0)  # naive, no tzinfo
+    with raises(ValueError):
+        await client.create(**params)
+
+
+async def test_create_past_datetime_raises(
+    client: FakeClient, sample: Dispatch
+) -> None:
+    """Test that create() raises ValueError for a start_time in the past."""
+    microgrid_id = MicrogridId(random.randint(1, 100))
+    params = to_create_params(microgrid_id, sample)
+    params["start_time"] = datetime(2000, 1, 1, tzinfo=timezone.utc)
+    with raises(ValueError):
+        await client.create(**params)