diff --git a/changelog.d/20230918_134911_rra_DM_40744.md b/changelog.d/20230918_134911_rra_DM_40744.md
new file mode 100644
index 00000000..a88ab71b
--- /dev/null
+++ b/changelog.d/20230918_134911_rra_DM_40744.md
@@ -0,0 +1,3 @@
+### Bug fixes
+
+- `safir.pydantic.normalize_datetime` now explicitly rejects input other than seconds since epoch or datetime objects with a validation error rather than attempting to treat the input as a datetime object and potentially throwing more obscure errors.
diff --git a/src/safir/pydantic.py b/src/safir/pydantic.py
index 7abdfc98..8bed00d7 100644
--- a/src/safir/pydantic.py
+++ b/src/safir/pydantic.py
@@ -20,18 +20,18 @@
 ]
 
 
-def normalize_datetime(v: int | datetime | None) -> datetime | None:
+def normalize_datetime(v: Any) -> datetime | None:
     """Pydantic field validator for datetime fields.
 
-    Supports `~datetime.datetime` fields given in either any format supported
-    by Pydantic natively, or in seconds since epoch (which Pydantic doesn't
-    support).  This field validator ensures that datetimes are always stored
-    in the model as timezone-aware UTC datetimes.
+    Supports `~datetime.datetime` fields given as either datetime objects or
+    seconds since epoch (not the other types Pydantic natively supports) and
+    ensures that the resulting datetime object is timezone-aware and in the
+    UTC timezone.
 
     Parameters
     ----------
     v
-        The field representing a `~datetime.datetime`.
+        Field representing a `~datetime.datetime`.
 
     Returns
     -------
@@ -61,6 +61,8 @@ class Info(BaseModel):
         return v
     elif isinstance(v, int):
         return datetime.fromtimestamp(v, tz=UTC)
+    elif not isinstance(v, datetime):
+        raise ValueError("Must be a datetime or seconds since epoch")
     elif v.tzinfo and v.tzinfo.utcoffset(v) is not None:
         return v.astimezone(UTC)
     else:
diff --git a/tests/pydantic_test.py b/tests/pydantic_test.py
index 70a7037f..4fad7b46 100644
--- a/tests/pydantic_test.py
+++ b/tests/pydantic_test.py
@@ -6,7 +6,12 @@
 from datetime import UTC, datetime, timedelta, timezone
 
 import pytest
-from pydantic import BaseModel, ValidationError, model_validator
+from pydantic import (
+    BaseModel,
+    ValidationError,
+    field_validator,
+    model_validator,
+)
 
 from safir.pydantic import (
     CamelCaseModel,
@@ -18,39 +23,55 @@
 
 
 def test_normalize_datetime() -> None:
-    assert normalize_datetime(None) is None
+    class TestModel(BaseModel):
+        time: datetime | None
+
+        _val = field_validator("time", mode="before")(normalize_datetime)
+
+    assert TestModel(time=None).time is None
 
     date = datetime.fromtimestamp(1668814932, tz=UTC)
-    assert normalize_datetime(1668814932) == date
+    model = TestModel(time=1668814932)  # type: ignore[arg-type]
+    assert model.time == date
 
     mst_zone = timezone(-timedelta(hours=7))
     mst_date = datetime.now(tz=mst_zone)
     utc_date = mst_date.astimezone(UTC)
-    assert normalize_datetime(mst_date) == utc_date
+    assert TestModel(time=mst_date).time == utc_date
 
     naive_date = datetime.utcnow()  # noqa: DTZ003
-    aware_date = normalize_datetime(naive_date)
+    aware_date = TestModel(time=naive_date).time
     assert aware_date == naive_date.replace(tzinfo=UTC)
     assert aware_date.tzinfo == UTC
 
+    with pytest.raises(ValueError, match=r"Must be a datetime or seconds .*"):
+        TestModel(time="2023-01-25T15:44:00+00:00")  # type: ignore[arg-type]
+
 
 def test_normalize_isodatetime() -> None:
-    assert normalize_isodatetime(None) is None
+    class TestModel(BaseModel):
+        time: datetime | None
+
+        _val = field_validator("time", mode="before")(normalize_isodatetime)
+
+    assert TestModel(time=None).time is None
 
     date = datetime.fromisoformat("2023-01-25T15:44:34+00:00")
-    assert date == normalize_isodatetime("2023-01-25T15:44:34Z")
+    model = TestModel(time="2023-01-25T15:44:34Z")  # type: ignore[arg-type]
+    assert model.time == date
 
     date = datetime.fromisoformat("2023-01-25T15:44:00+00:00")
-    assert date == normalize_isodatetime("2023-01-25T15:44Z")
+    model = TestModel(time="2023-01-25T15:44Z")  # type: ignore[arg-type]
+    assert model.time == date
 
     with pytest.raises(ValueError, match=r"Must be a string in .* format"):
-        normalize_isodatetime("2023-01-25T15:44:00+00:00")
+        TestModel(time="2023-01-25T15:44:00+00:00")  # type: ignore[arg-type]
 
     with pytest.raises(ValueError, match=r"Must be a string in .* format"):
-        normalize_isodatetime(1668814932)  # type: ignore[arg-type]
+        TestModel(time=1668814932)  # type: ignore[arg-type]
 
     with pytest.raises(ValueError, match=r"Must be a string in .* format"):
-        normalize_isodatetime("next thursday")
+        TestModel(time="next thursday")  # type: ignore[arg-type]
 
 
 def test_to_camel_case() -> None: