diff --git a/backend/tests/conftest.py b/backend/tests/conftest.py
index ba9f4f6d..0aca6420 100644
--- a/backend/tests/conftest.py
+++ b/backend/tests/conftest.py
@@ -337,20 +337,45 @@ class _NeverBack(_dt.datetime):
         return t
 
 
+_last_signature_second = [0]
+
+
+def _signature_second_never_back(self) -> int:
+    """The session cookie's signing clock (itsdangerous, whole seconds), never going back: it refuses
+    a signature from the future, so a step back across a second boundary refused the cookie a
+    sign-in had just set. Read through the module at call time, so a probe can step it."""
+    import itsdangerous.timed
+
+    t = max(int(itsdangerous.timed.time.time()), _last_signature_second[0])
+    _last_signature_second[0] = t
+    return t
+
+
 @pytest.fixture(autouse=True)
 def _clock_never_goes_back(monkeypatch):
     """Rows a test makes one after another are stamped in that order. The wall clock of the docker
     VM the suite runs in steps BACK now and then (measured: 2 steps in 20 s, up to 0.5 s), and a
     test that makes three files and reads them newest first then got them out of order. Every loaded
     `app.*` module that took `datetime` from the datetime module reads this clock instead (a sweep,
-    as the limiter reset below does, so a new module is covered the day it lands). A database
+    as the limiter reset below does, so a new module is covered the day it lands) — and so does every
+    test module, so a test comparing a stamp the app wrote with "now" reads the same clock. The
+    session cookie's signature has a clock of its own (`_signature_second_never_back`). A database
     default (`server_default=func.now()`) is Postgres's clock and is not covered: a test that orders
     such rows made within a second of each other gives them distinct times itself."""
     import sys
 
+    from itsdangerous import TimestampSigner
+
+    tests_dir = os.path.dirname(os.path.abspath(__file__))
     for name, mod in list(sys.modules.items()):
-        if (name == "app" or name.startswith("app.")) and vars(mod).get("datetime") is _dt.datetime:
+        if vars(mod).get("datetime") is not _dt.datetime:
+            continue
+        mod_file = getattr(mod, "__file__", None) or ""
+        if name == "app" or name.startswith("app.") or mod_file.startswith(tests_dir + os.sep):
             monkeypatch.setattr(mod, "datetime", _NeverBack)
+    # Per test: a cookie never outlives its test, and a probe's stepped clock must not carry over.
+    _last_signature_second[0] = 0
+    monkeypatch.setattr(TimestampSigner, "get_timestamp", _signature_second_never_back)
 
 
 @pytest.fixture(autouse=True)
diff --git a/backend/tests/test_test_clock.py b/backend/tests/test_test_clock.py
new file mode 100644
index 00000000..71a599fa
--- /dev/null
+++ b/backend/tests/test_test_clock.py
@@ -0,0 +1,51 @@
+"""The suite's clock (conftest `_clock_never_goes_back`), DB-lane.
+
+The wall clock of the docker VM the suite runs in steps BACK now and then (measured: up to 0.5 s).
+These hold the two places that read it besides the app: the session cookie's signature, and a test
+comparing a stamp the app wrote with "now".
+"""
+
+import time
+from datetime import datetime, timezone
+from types import SimpleNamespace
+
+import itsdangerous.timed
+
+_PW = "correct horse battery"
+
+
+def test_a_cookie_signed_just_before_the_clock_steps_back_still_signs_in(
+    client, make_user, monkeypatch
+):
+    # The signature carries whole seconds and itsdangerous refuses one from the future: a step back
+    # across a second boundary refused the cookie a sign-in had just set (a 401 at random).
+    clock = [int(time.time()) + 0.2]
+    monkeypatch.setattr(itsdangerous.timed, "time", SimpleNamespace(time=lambda: clock[0]))
+    make_user(email="clock@example.com", password=_PW)
+    r = client.post("/auth/password-login", json={"email": "clock@example.com", "password": _PW})
+    assert r.status_code == 200
+    clock[0] -= 0.5
+    assert client.get("/api/me/notifications").status_code == 200
+
+
+def test_a_test_reads_the_clock_the_app_stamps_with(monkeypatch):
+    # A test asserting "the app's stamp is not in the future" compared it with the wall clock, which
+    # may have stepped back since the app read its own.
+    import conftest
+    from app import diagnostics
+
+    # The app reads 12:00:01, then the wall clock steps back half a second (a time past any real one,
+    # so a test reading the wall clock instead is behind the stamp).
+    later = datetime(2099, 1, 1, 12, 0, 1, tzinfo=timezone.utc)
+    stepped_back = datetime(2099, 1, 1, 12, 0, 0, 500000, tzinfo=timezone.utc)
+    readings = iter([later])
+
+    class Stepping(datetime):
+        @classmethod
+        def now(cls, tz=None):
+            return next(readings, stepped_back)
+
+    monkeypatch.setattr(conftest._NeverBack, "clock", Stepping)
+    monkeypatch.setattr(conftest._NeverBack, "last", None)
+    stamped = diagnostics.datetime.now(timezone.utc)
+    assert datetime.now(timezone.utc) >= stamped