diff --git a/CONTRIBUTING.md b/CONTRIBUTING.md
index f6ba7a3f..511c0c62 100644
--- a/CONTRIBUTING.md
+++ b/CONTRIBUTING.md
@@ -86,6 +86,18 @@ Tests can be run with `tox`. Some basic code style tests can be run with `tox
 -e flake8` and most other targets run the test suite with various supported
 Python interpreters.
 
+Tests use the real date by default. For reproducible builds, set
+`STDNUM_TEST_DATE` to an explicit date in `YYYY-MM-DD` format, for example
+`STDNUM_TEST_DATE=2026-09-21 tox -e py312`. The same variable works with a
+direct `pytest` invocation. Invalid or empty values fail with a configuration
+error.
+
+This test-only option replaces `datetime.date.today()` during the test session;
+it does not change the host clock or the installed library. Date construction
+still returns ordinary date objects. Choose a reference date appropriate for
+the release being tested. Leave the variable unset for development and
+scheduled tests so that changes caused by the passage of time remain visible.
+
 Module implementations have a couple of smaller test cases that also serve as
 basic documentation of the happy flow.
 
diff --git a/conftest.py b/conftest.py
new file mode 100644
index 00000000..1038c25a
--- /dev/null
+++ b/conftest.py
@@ -0,0 +1,72 @@
+# conftest.py - optional reference date for tests
+#
+# Copyright (C) 2026 Ryan Duguid
+#
+# This library is free software; you can redistribute it and/or
+# modify it under the terms of the GNU Lesser General Public
+# License as published by the Free Software Foundation; either
+# version 2.1 of the License, or (at your option) any later version.
+#
+# This library is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+# Lesser General Public License for more details.
+#
+# You should have received a copy of the GNU Lesser General Public
+# License along with this library; if not, see <https://www.gnu.org/licenses/>.
+
+"""Pytest configuration for python-stdnum."""
+
+import datetime
+import os
+from unittest.mock import patch
+
+import pytest
+
+
+pytest_plugins = ['pytester']
+
+
+def pytest_configure(config):
+    """Validate the optional reference date before collecting tests."""
+    value = os.environ.get('STDNUM_TEST_DATE')
+    config._stdnum_reference_date = None
+    if value is None:
+        return
+    try:
+        fixed_date = datetime.date.fromisoformat(value)
+        if fixed_date.isoformat() != value:
+            raise ValueError()
+    except ValueError:
+        raise pytest.UsageError('STDNUM_TEST_DATE must be a valid date in YYYY-MM-DD format')
+    config._stdnum_reference_date = fixed_date
+
+
+@pytest.fixture(scope='session', autouse=True)
+def reference_date(request):
+    """Optionally replace date.today() for the duration of the test session."""
+    fixed_date = request.config._stdnum_reference_date
+    if fixed_date is None:
+        yield
+        return
+
+    real_date = datetime.date
+
+    class DateMeta(type):
+        """Keep isinstance checks compatible with ordinary date objects."""
+
+        def __instancecheck__(cls, instance):
+            return isinstance(instance, real_date)
+
+    class ReferenceDate(real_date, metaclass=DateMeta):
+        """Return ordinary dates, overriding only the current date."""
+
+        def __new__(cls, *args, **kwargs):
+            return real_date(*args, **kwargs)
+
+        @classmethod
+        def today(cls):
+            return fixed_date
+
+    with patch.object(datetime, 'date', ReferenceDate):
+        yield
diff --git a/tests/test_reference_date.py b/tests/test_reference_date.py
new file mode 100644
index 00000000..4395db63
--- /dev/null
+++ b/tests/test_reference_date.py
@@ -0,0 +1,78 @@
+# test_reference_date.py - test the optional reference date
+#
+# Copyright (C) 2026 Ryan Duguid
+#
+# This library is free software; you can redistribute it and/or
+# modify it under the terms of the GNU Lesser General Public
+# License as published by the Free Software Foundation; either
+# version 2.1 of the License, or (at your option) any later version.
+#
+# This library is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+# Lesser General Public License for more details.
+#
+# You should have received a copy of the GNU Lesser General Public
+# License along with this library; if not, see <https://www.gnu.org/licenses/>.
+
+"""Check the test reference date in isolated pytest processes."""
+
+from pathlib import Path
+
+import pytest
+
+
+@pytest.fixture
+def clock_suite(pytester: pytest.Pytester, monkeypatch: pytest.MonkeyPatch) -> pytest.Pytester:
+    """Install the real configuration in an isolated test directory."""
+    # These child suites test pytest configuration, not library coverage.
+    monkeypatch.delenv('COV_CORE_DATAFILE', raising=False)
+    source = Path(__file__).resolve().parent.parent / 'conftest.py'
+    pytester.makeconftest(source.read_text() + """
+import datetime
+_original_date = datetime.date
+
+def pytest_sessionfinish(session):
+    assert datetime.date is _original_date
+""")
+    pytester.makepyfile("""
+import datetime
+import os
+
+real_date = datetime.date
+real_today = datetime.date.today()
+
+def test_date():
+    value = os.environ.get('STDNUM_TEST_DATE')
+    expected = real_date.fromisoformat(value) if value is not None else real_today
+    assert datetime.date.today() == expected
+    assert datetime.date(2000, 2, 29) == real_date(2000, 2, 29)
+    assert repr(datetime.date(2000, 2, 29)) == 'datetime.date(2000, 2, 29)'
+    assert isinstance(real_date(2000, 2, 29), datetime.date)
+    assert isinstance(datetime.datetime.now().date(), datetime.date)
+    if value is None:
+        assert datetime.date is real_date
+""")
+    return pytester
+
+
+def test_default_uses_real_date(clock_suite: pytest.Pytester, monkeypatch: pytest.MonkeyPatch) -> None:
+    """No reference date leaves the date class and clock unchanged."""
+    monkeypatch.delenv('STDNUM_TEST_DATE', raising=False)
+    clock_suite.runpytest_subprocess('-q').assert_outcomes(passed=1)
+
+
+@pytest.mark.parametrize('value', ['2024-01-01', '2040-04-24', '2024-02-29'])
+def test_explicit_reference_date(clock_suite: pytest.Pytester, monkeypatch: pytest.MonkeyPatch, value: str) -> None:
+    """A requested date applies during tests and is restored afterwards."""
+    monkeypatch.setenv('STDNUM_TEST_DATE', value)
+    clock_suite.runpytest_subprocess('-q').assert_outcomes(passed=1)
+
+
+@pytest.mark.parametrize('value', ['', 'invalid', '2023-02-29', '20240101'])
+def test_invalid_reference_date(clock_suite: pytest.Pytester, monkeypatch: pytest.MonkeyPatch, value: str) -> None:
+    """Invalid dates fail with an actionable configuration error."""
+    monkeypatch.setenv('STDNUM_TEST_DATE', value)
+    result = clock_suite.runpytest_subprocess('-q')
+    assert result.ret == pytest.ExitCode.USAGE_ERROR
+    assert 'STDNUM_TEST_DATE must be a valid date in YYYY-MM-DD format' in result.stderr.str()
diff --git a/tox.ini b/tox.ini
index fec260d4..4676e83c 100644
--- a/tox.ini
+++ b/tox.ini
@@ -6,6 +6,7 @@ skip_missing_interpreters = true
 deps = pytest
        pytest-cov
 commands = pytest
+passenv = STDNUM_TEST_DATE
 setenv=
     PYTHONWARNINGS=all
 
@@ -33,6 +34,7 @@ setenv=
 [testenv:mypy]
 skip_install = true
 deps = mypy<2.0  # required to keep Python 3.9 support
+       pytest<9  # pytest 9 syntax exceeds the Python 3.9 type-check target
        types-requests
        zeep
 commands =