diff --git a/pyproject.toml b/pyproject.toml
index 99efdeb..f5a1391 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -43,6 +43,9 @@ call = "spoolman.bump:bump"
 [tool.pdm.scripts.app]
 cmd = "uvicorn spoolman.main:app"
 
+[tool.pdm.scripts.itest]
+cmd = "python tests_integration/run.py"
+
 [tool.ruff]
 select = ["ALL"]
 ignore = [
diff --git a/spoolman/api/v1/models.py b/spoolman/api/v1/models.py
index ba4b051..7075692 100644
--- a/spoolman/api/v1/models.py
+++ b/spoolman/api/v1/models.py
@@ -1,14 +1,31 @@
 """Pydantic data models for typing the FastAPI request/responses."""
 
-from datetime import datetime
+from datetime import datetime, timezone
 from typing import Optional
 
-from pydantic import BaseModel, Field
+from pydantic import BaseModel as PydanticBaseModel
+from pydantic import Field
 
 from spoolman.database import models
 from spoolman.math import length_from_weight
 
 
+def datetime_to_str(dt: datetime) -> str:
+    """Convert a datetime object to a string."""
+    if dt.tzinfo is None:
+        dt = dt.astimezone(tz=timezone.utc)
+    return dt.isoformat().replace("+00:00", "Z")
+
+
+class BaseModel(PydanticBaseModel):
+    class Config:
+        """Pydantic configuration."""
+
+        json_encoders = {  # noqa: RUF012
+            datetime: datetime_to_str,
+        }
+
+
 class Message(BaseModel):
     message: str = Field()
 
diff --git a/tests_integration/tests/spool/test_add.py b/tests_integration/tests/spool/test_add.py
index dfb7603..a64b0e1 100644
--- a/tests_integration/tests/spool/test_add.py
+++ b/tests_integration/tests/spool/test_add.py
@@ -50,8 +50,8 @@ def test_add_spool_remaining_weight(random_filament: dict[str, Any]):
     assert spool == {
         "id": spool["id"],
         "registered": spool["registered"],
-        "first_used": "2023-01-02T11:00:00",
-        "last_used": "2023-01-02T11:00:00",
+        "first_used": "2023-01-02T11:00:00Z",
+        "last_used": "2023-01-02T11:00:00Z",
         "filament": random_filament,
         "remaining_weight": pytest.approx(remaining_weight),
         "used_weight": pytest.approx(used_weight),
@@ -70,8 +70,8 @@ def test_add_spool_remaining_weight(random_filament: dict[str, Any]):
 def test_add_spool_used_weight(random_filament: dict[str, Any]):
     """Test adding a spool to the database."""
     # Execute
-    first_used = "2023-01-01T00:00:00"
-    last_used = "2023-01-02T00:00:00"
+    first_used = "2023-01-01T00:00:00Z"
+    last_used = "2023-01-02T00:00:00Z"
     used_weight = 250
     location = "The Pantry"
     lot_nr = "123456789"
diff --git a/tests_integration/tests/spool/test_update.py b/tests_integration/tests/spool/test_update.py
index 56e3f45..c1d3be3 100644
--- a/tests_integration/tests/spool/test_update.py
+++ b/tests_integration/tests/spool/test_update.py
@@ -61,8 +61,8 @@ def test_update_spool(random_filament: dict[str, Any]):
     )
 
     spool = result.json()
-    assert spool["first_used"] == "2023-01-01T10:00:00"
-    assert spool["last_used"] == "2023-01-02T10:00:00"
+    assert spool["first_used"] == "2023-01-01T10:00:00Z"
+    assert spool["last_used"] == "2023-01-02T10:00:00Z"
     assert spool["remaining_weight"] == pytest.approx(remaining_weight)
     assert spool["used_weight"] == pytest.approx(used_weight)
     assert spool["remaining_length"] == pytest.approx(remaining_length)