diff --git a/README.rst b/README.rst
index 02e5e8f..5bfa218 100644
--- a/README.rst
+++ b/README.rst
@@ -351,6 +351,8 @@ set of tests running in one session. A session all of the tests
 that will be run with one invocation of pytest.
 
 A session timeout is set with ``--session-timeout`` and is in seconds.
+Elapsed session time uses a monotonic clock, so changes to the system clock
+do not affect the timeout.
 
 The following example shows a session timeout of 10 minutes (600 seconds)::
 
diff --git a/pytest_timeout.py b/pytest_timeout.py
index 192a16c..370dbe4 100644
--- a/pytest_timeout.py
+++ b/pytest_timeout.py
@@ -165,7 +165,7 @@ def pytest_configure(config):
         if ini:
             timeout = _validate_timeout(config.getini("session_timeout"), "config file")
     if timeout is not None:
-        expire_time = time.time() + timeout
+        expire_time = time.monotonic() + timeout
     else:
         expire_time = 0
         timeout = 0
@@ -192,7 +192,7 @@ def pytest_runtest_protocol(item):
 
     #  check session timeout
     expire_time = item.session.config.stash[SESSION_EXPIRE_KEY]
-    if expire_time and (expire_time < time.time()):
+    if expire_time and (expire_time < time.monotonic()):
         timeout = item.session.config.stash[SESSION_TIMEOUT_KEY]
         item.session.shouldfail = f"session-timeout: {timeout} sec exceeded"
 
diff --git a/test_pytest_timeout.py b/test_pytest_timeout.py
index ff36c40..c7f6ad3 100644
--- a/test_pytest_timeout.py
+++ b/test_pytest_timeout.py
@@ -708,6 +708,46 @@ def test_two(slow_setup_and_teardown):
     result.assert_outcomes(passed=1)
 
 
+@pytest.mark.parametrize(
+    ("wall_shift", "elapsed", "passed"), [(1000, 1, 2), (-1000, 11, 1)]
+)
+def test_session_timeout_ignores_wall_clock_changes(
+    pytester, wall_shift, elapsed, passed
+):
+    pytester.makeconftest(
+        """
+        from types import SimpleNamespace
+        import pytest
+        import pytest_timeout
+
+        clock = SimpleNamespace(wall=100, elapsed=0)
+
+        @pytest.hookimpl(tryfirst=True)
+        def pytest_configure(config):
+            pytest_timeout.time = SimpleNamespace(
+                time=lambda: clock.wall,
+                monotonic=lambda: clock.elapsed,
+            )
+        """
+    )
+    pytester.makepyfile(
+        f"""
+        from conftest import clock
+
+        def test_one():
+            clock.wall += {wall_shift}
+            clock.elapsed += {elapsed}
+
+        def test_two():
+            pass
+        """
+    )
+    result = pytester.runpytest_subprocess("--session-timeout=10")
+    result.assert_outcomes(passed=passed)
+    if passed == 1:
+        result.stdout.fnmatch_lines(["*session-timeout: 10.0 sec exceeded*"])
+
+
 def test_ini_session_timeout(pytester):
     pytester.makepyfile(
         """