I haven't been able to find a way to mock the date.today() function
A possible solution is to use the freezgun library.

It looks like there's an example in the unittest.mock documentation that provides a way to mock date.today: https://docs.python.org/3/library/unittest.mock-examples.html#partial-mocking, which will require a separate test be written since I doubt it'll be possible to combine this approach with using parametrize()

@vdonato I tried this option and several options with a modified patch path

Unfortunately, everything falls with errors

For example this:

def test_min_max_values_missing_value(self):
    with patch("streamlit.delta_generator.DeltaGenerator.date_input.date") as mock_date:
        mock_date.today.return_value = date(1999, 10, 8)
        mock_date.side_effect = lambda *args, **kw: date(*args, **kw)

        st.date_input("the label", [])
        c = self.get_delta_from_queue().new_element.date_input
        self.assertEqual(c.min, "1989/10/08")
        self.assertEqual(c.max, "2009/10/08")

fails with error
AttributeError: <function TimeWidgetsMixin.date_input at 0x1526e48b0> does not have the attribute 'date'

I think we will find a way to mock dates in tests. Or we will start using a library for this.

For now, I will leave a TODO comment, to come back to this later.

Please pay attention to the docstring.

Do we need to update the documentation somewhere else?
Does this docstring line explain the behavior well enough, or is it better to rephrase?

The documentation should be updated automatically (as long as this docstring is already in the existing docs, which it is) since docs in our API reference are rendered from the docstrings themselves.

As for the documentation wording, I think this is fine, but it may be slightly clearer to phrase it as something like

"The minimum selectable date. If value is a date, defaults to value - 10 years. If value is the interval [start, end], defaults to start - 10 years."

It looks like there's an example in the unittest.mock documentation that provides a way to mock date.today: https://docs.python.org/3/library/unittest.mock-examples.html#partial-mocking, which will require a separate test be written since I doubt it'll be possible to combine this approach with using parametrize()

The documentation should be updated automatically (as long as this docstring is already in the existing docs, which it is) since docs in our API reference are rendered from the docstrings themselves.

As for the documentation wording, I think this is fine, but it may be slightly clearer to phrase it as something like

"The minimum selectable date. If value is a date, defaults to value - 10 years. If value is the interval [start, end], defaults to start - 10 years."