diff --git a/airflow-core/tests/unit/api_fastapi/core_api/routes/public/test_dag_run.py b/airflow-core/tests/unit/api_fastapi/core_api/routes/public/test_dag_run.py
index ea3523fb07535..ae89fe1e4b172 100644
--- a/airflow-core/tests/unit/api_fastapi/core_api/routes/public/test_dag_run.py
+++ b/airflow-core/tests/unit/api_fastapi/core_api/routes/public/test_dag_run.py
@@ -2246,7 +2246,7 @@ def test_should_respond_400_if_manual_runs_denied(self, test_client, session, te
     def test_should_response_409_for_duplicate_logical_date(self, test_client):
         RUN_ID_1 = "random_1"
         RUN_ID_2 = "random_2"
-        now = timezone.utcnow().isoformat().replace("+00:00", "Z")
+        now = from_datetime_to_zulu(timezone.utcnow())
         note = "duplicate logical date test"
         response_1 = test_client.post(
             f"/dags/{DAG1_ID}/dagRuns",
diff --git a/airflow-core/tests/unit/api_fastapi/core_api/routes/public/test_hitl.py b/airflow-core/tests/unit/api_fastapi/core_api/routes/public/test_hitl.py
index e9ed732c67d4a..eabbb0a7559d2 100644
--- a/airflow-core/tests/unit/api_fastapi/core_api/routes/public/test_hitl.py
+++ b/airflow-core/tests/unit/api_fastapi/core_api/routes/public/test_hitl.py
@@ -629,7 +629,7 @@ def test_should_respond_200_with_existing_response_and_concrete_query(
                     "multiple": False,
                     "params": {"input_1": {"value": 1, "schema": {}, "description": None}},
                     "assigned_users": [],
-                    "created_at": DEFAULT_CREATED_AT.isoformat().replace("+00:00", "Z"),
+                    "created_at": from_datetime_to_zulu_without_ms(DEFAULT_CREATED_AT),
                     "responded_by_user": None,
                     "responded_at": None,
                     "chosen_options": None,
diff --git a/airflow-core/tests/unit/api_fastapi/execution_api/versions/v2026_04_06/test_task_instances.py b/airflow-core/tests/unit/api_fastapi/execution_api/versions/v2026_04_06/test_task_instances.py
index a914ac6c6e563..d855291727756 100644
--- a/airflow-core/tests/unit/api_fastapi/execution_api/versions/v2026_04_06/test_task_instances.py
+++ b/airflow-core/tests/unit/api_fastapi/execution_api/versions/v2026_04_06/test_task_instances.py
@@ -24,6 +24,7 @@
 from airflow.utils.state import DagRunState, State
 
 from tests_common.test_utils.db import clear_db_runs
+from tests_common.test_utils.format_datetime import from_datetime_to_zulu_without_ms
 
 pytestmark = pytest.mark.db_test
 
@@ -125,7 +126,7 @@ def test_old_version_preserves_real_start_date(
 
         assert response.status_code == 200
         assert dag_run["start_date"] is not None, "start_date should not be None when DagRun has started"
-        assert dag_run["start_date"] == TIMESTAMP.isoformat().replace("+00:00", "Z")
+        assert dag_run["start_date"] == from_datetime_to_zulu_without_ms(TIMESTAMP)
 
 
 class TestNextKwargsBackwardCompat: