The documentation for the `%S` formatter suggests that it supports leap seconds, but the footnote contradicts that:

> The range really is 0 to 61; according to the Posix standard this
> accounts for leap seconds and the (very rare) double leap seconds.
> The time module may produce and does accept leap seconds since it is
> based on the Posix standard, but the datetime module does not accept
> leap seconds in strptime() input nor will it produce them in
> strftime() output.

I propose that the documentation be updated to mirror the 3.X documentation: no leap second in the example and a footnote which makes note of that and suggests using the time module.

Here's the relevant diff. Will commit on a +1.

diff --git a/Doc/library/datetime.rst b/Doc/library/datetime.rst
--- a/Doc/library/datetime.rst
+++ b/Doc/library/datetime.rst
@@ -1664,7 +1664,7 @@
 | ``%M``    | Minute as a zero-padded        | 00, 01, ..., 59        |       |
 |           | decimal number.                |                        |       |
 +-----------+--------------------------------+------------------------+-------+
-| ``%S``    | Second as a zero-padded        | 00, 01, ..., 61        | \(3)  |
+| ``%S``    | Second as a zero-padded        | 00, 01, ..., 59        | \(3)  |
 |           | decimal number.                |                        |       |
 +-----------+--------------------------------+------------------------+-------+
 | ``%f``    | Microsecond as a decimal       | 000000, 000001, ...,   | \(4)  |
@@ -1728,12 +1728,8 @@
    the output hour field if the ``%I`` directive is used to parse the hour.
 
 (3)
-   The range really is ``0`` to ``61``; according to the Posix standard this
-   accounts for leap seconds and the (very rare) double leap seconds.
-   The :mod:`time` module may produce and does accept leap seconds since
-   it is based on the Posix standard, but the :mod:`datetime` module
-   does not accept leap seconds in :meth:`strptime` input nor will it
-   produce them in :func:`strftime` output.
+   Unlike the :mod:`time` module, the :mod:`datetime` module does not support
+   leap seconds.
 
 (4)
    ``%f`` is an extension to the set of format characters in the C standard

+1

:-)

New changeset 5063dab96843 by David Wolever in branch '2.7':
Issue #18718: datetime documentation contradictory on leap second support
http://hg.python.org/cpython/rev/5063dab96843

Can this be closed?

D'oh, yes, it can (and I've closed it now).