diff --git a/airflow-core/src/airflow/dag_processing/processor.py b/airflow-core/src/airflow/dag_processing/processor.py
index 1134c673da725..64b84ab46c8ae 100644
--- a/airflow-core/src/airflow/dag_processing/processor.py
+++ b/airflow-core/src/airflow/dag_processing/processor.py
@@ -19,6 +19,7 @@
 import functools
 import os
 import sys
+import time
 import traceback
 from pathlib import Path
 from typing import TYPE_CHECKING, Annotated, BinaryIO, Callable, ClassVar, Literal, Union
@@ -229,6 +230,8 @@ class DagFileProcessorProcess(WatchedSubprocess):
     logger_filehandle: BinaryIO
     parsing_result: DagFileParsingResult | None = None
     decoder: ClassVar[TypeAdapter[ToManager]] = TypeAdapter[ToManager](ToManager)
+    _start_time: float | None = None
+    """Time when the process started parsing the file."""
 
     @classmethod
     def start(  # type: ignore[override]
@@ -242,6 +245,7 @@ def start(  # type: ignore[override]
     ) -> Self:
         proc: Self = super().start(target=target, **kwargs)
         proc._on_child_started(callbacks, path, bundle_path)
+        proc._start_time = time.time()
         return proc
 
     def _on_child_started(
@@ -305,7 +309,18 @@ def is_ready(self) -> bool:
 
     @property
     def start_time(self) -> float:
-        return self._process.create_time()
+        if not self._start_time:
+            raise ValueError("start_time not set")
+        return self._start_time
+
+    @start_time.setter
+    def start_time(self, value: float | None = None) -> None:
+        if self._start_time:
+            # Ensures this is not set twice
+            return
+        if not value:
+            value = time.time()
+        self._start_time = value
 
     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 027d58e6fa1a9..ad0d82070d5b2 100644
--- a/airflow-core/tests/unit/dag_processing/test_manager.py
+++ b/airflow-core/tests/unit/dag_processing/test_manager.py
@@ -26,7 +26,6 @@
 import shutil
 import signal
 import textwrap
-import time
 from collections import deque
 from datetime import datetime, timedelta
 from logging.config import dictConfig
@@ -135,7 +134,6 @@ def teardown_class(self):
     def mock_processor(self) -> DagFileProcessorProcess:
         proc = MagicMock()
         logger_filehandle = MagicMock()
-        proc.create_time.return_value = time.time()
         proc.wait.return_value = 0
         ret = DagFileProcessorProcess(
             process_log=MagicMock(),
@@ -493,7 +491,7 @@ 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._start_time = timezone.make_aware(datetime.min).timestamp()
         manager._processors = {
             DagFileInfo(
                 bundle_name="testing", rel_path=Path("abc.txt"), bundle_path=TEST_DAGS_FOLDER
@@ -512,7 +510,7 @@ def test_kill_timed_out_processors_no_kill(self):
         )
 
         processor = self.mock_processor()
-        processor._process.create_time.return_value = timezone.make_aware(datetime.max).timestamp()
+        processor._start_time = timezone.make_aware(datetime.max).timestamp()
         manager._processors = {
             DagFileInfo(
                 bundle_name="testing", rel_path=Path("abc.txt"), bundle_path=TEST_DAGS_FOLDER
@@ -522,6 +520,21 @@ def test_kill_timed_out_processors_no_kill(self):
             manager._kill_timed_out_processors()
         mock_kill.assert_not_called()
 
+    def test_start_time_raises_when_not_set(self):
+        manager = DagFileProcessorManager(
+            max_runs=1,
+            processor_timeout=5,
+        )
+
+        processor = self.mock_processor()
+        manager._processors = {
+            DagFileInfo(
+                bundle_name="testing", rel_path=Path("abc.txt"), bundle_path=TEST_DAGS_FOLDER
+            ): processor
+        }
+        with pytest.raises(ValueError, match="start_time not set"):
+            manager._kill_timed_out_processors()
+
     @pytest.mark.usefixtures("testing_dag_bundle")
     @pytest.mark.parametrize(
         ["callbacks", "path", "expected_buffer"],