diff --git a/src/openstack_billing_db/billing.py b/src/openstack_billing_db/billing.py
index a93f02b..6eb9ca3 100644
--- a/src/openstack_billing_db/billing.py
+++ b/src/openstack_billing_db/billing.py
@@ -94,8 +94,8 @@ def get_runtime_for_instance(
     runtime = instance.get_runtime_during(start, end)
     for interval_start, interval_end in excluded_intervals:
         excluded_runtime = instance.get_runtime_during(
-            start_time=interval_start,
-            end_time=interval_end,
+            start_time=interval_start.replace(tzinfo=None),
+            end_time=interval_end.replace(tzinfo=None),
         )
         runtime = runtime - excluded_runtime
 
diff --git a/src/openstack_billing_db/tests/unit/test_billing.py b/src/openstack_billing_db/tests/unit/test_billing.py
index 9e975df..9c20f76 100644
--- a/src/openstack_billing_db/tests/unit/test_billing.py
+++ b/src/openstack_billing_db/tests/unit/test_billing.py
@@ -1,5 +1,5 @@
 import uuid
-from datetime import datetime, timedelta
+from datetime import datetime, timedelta, timezone
 import pytest
 
 from openstack_billing_db import billing
@@ -36,6 +36,49 @@ def test_instance_simple_runtime():
     assert r.total_seconds_stopped == 0
 
 
+def test_instance_runtime_timezone_mismatch():
+    """Test that mismatched timezones between start/end and excluded_intervals raises an error."""
+    time = datetime(year=2000, month=1, day=1, hour=0, minute=0, second=0)
+    events = [
+        InstanceEvent(time=time, name="create", message=""),
+        InstanceEvent(time=time + timedelta(days=15), name="delete", message=""),
+    ]
+    i = Instance(
+        uuid=uuid.uuid4().hex, name=uuid.uuid4().hex, flavor=FLAVORS[1], events=events
+    )
+
+    # Start and end are timezone-naive
+    start = datetime(year=2000, month=1, day=1, hour=0, minute=0, second=0)
+    end = datetime(year=2000, month=2, day=1, hour=0, minute=0, second=0)
+
+    # excluded_intervals are timezone-aware
+    excluded_intervals = [
+        (
+            datetime(
+                year=2000,
+                month=1,
+                day=7,
+                hour=0,
+                minute=0,
+                second=0,
+                tzinfo=timezone.utc,
+            ),
+            datetime(
+                year=2000,
+                month=1,
+                day=8,
+                hour=0,
+                minute=0,
+                second=0,
+                tzinfo=timezone.utc,
+            ),
+        ),
+    ]
+
+    # Runs without error
+    billing.get_runtime_for_instance(i, start, end, excluded_intervals)
+
+
 def test_billing_add_su_hours():
     invoice = billing.ProjectInvoice(
         project_name="foo",