diff --git a/airflow-core/src/airflow/dag_processing/manager.py b/airflow-core/src/airflow/dag_processing/manager.py
index b8ddf688dcb2a..324813cec3828 100644
--- a/airflow-core/src/airflow/dag_processing/manager.py
+++ b/airflow-core/src/airflow/dag_processing/manager.py
@@ -992,7 +992,7 @@ def prepare_file_queue(self, known_files: dict[str, set[DagFileInfo]]):
 
     def _kill_timed_out_processors(self):
         """Kill any file processors that timeout to defend against process hangs."""
-        now = time.time()
+        now = time.monotonic()
         processors_to_remove = []
         for file, processor in self._processors.items():
             duration = now - processor.start_time
diff --git a/airflow-core/src/airflow/dag_processing/processor.py b/airflow-core/src/airflow/dag_processing/processor.py
index 25f9e2a73ed87..ef3e5cb69ca56 100644
--- a/airflow-core/src/airflow/dag_processing/processor.py
+++ b/airflow-core/src/airflow/dag_processing/processor.py
@@ -312,9 +312,5 @@ def is_ready(self) -> bool:
 
         return self._num_open_sockets == 0
 
-    @property
-    def start_time(self) -> float:
-        return self._process.create_time()
-
     def wait(self) -> int:
         raise NotImplementedError(f"Don't call wait on {type(self).__name__} objects")
diff --git a/airflow-core/tests/unit/dag_processing/test_manager.py b/airflow-core/tests/unit/dag_processing/test_manager.py
index 3c180afa76b80..ebf684c678843 100644
--- a/airflow-core/tests/unit/dag_processing/test_manager.py
+++ b/airflow-core/tests/unit/dag_processing/test_manager.py
@@ -133,7 +133,7 @@ def teardown_class(self):
         clear_db_import_errors()
         clear_db_dag_bundles()
 
-    def mock_processor(self) -> tuple[DagFileProcessorProcess, socket]:
+    def mock_processor(self, start_time: float | None = None) -> tuple[DagFileProcessorProcess, socket]:
         proc = MagicMock()
         logger_filehandle = MagicMock()
         proc.create_time.return_value = time.time()
@@ -148,6 +148,8 @@ def mock_processor(self) -> tuple[DagFileProcessorProcess, socket]:
             requests_fd=123,
             logger_filehandle=logger_filehandle,
         )
+        if start_time:
+            ret.start_time = start_time
         ret._num_open_sockets = 0
         return ret, read_end
 
@@ -518,9 +520,7 @@ def test_scan_stale_dags(self, testing_dag_bundle):
 
     def test_kill_timed_out_processors_kill(self):
         manager = DagFileProcessorManager(max_runs=1, processor_timeout=5)
-
-        processor, _ = self.mock_processor()
-        processor._process.create_time.return_value = timezone.make_aware(datetime.min).timestamp()
+        processor, _ = self.mock_processor(start_time=16000)
         manager._processors = {
             DagFileInfo(
                 bundle_name="testing", rel_path=Path("abc.txt"), bundle_path=TEST_DAGS_FOLDER
diff --git a/task-sdk/src/airflow/sdk/execution_time/supervisor.py b/task-sdk/src/airflow/sdk/execution_time/supervisor.py
index db9474e68d569..0b77156578bb0 100644
--- a/task-sdk/src/airflow/sdk/execution_time/supervisor.py
+++ b/task-sdk/src/airflow/sdk/execution_time/supervisor.py
@@ -428,6 +428,9 @@ class WatchedSubprocess:
     subprocess_logs_to_stdout: bool = False
     """Duplicate log messages to stdout, or only send them to ``self.process_log``."""
 
+    start_time: float = attrs.field(factory=time.monotonic)
+    """The start time of the child process."""
+
     @classmethod
     def start(
         cls,
@@ -481,6 +484,7 @@ def start(
             process=psutil.Process(pid),
             requests_fd=requests_fd,
             process_log=logger,
+            start_time=time.monotonic(),
             **constructor_kwargs,
         )