Insertion of each UTC leap second is usually decided about six months in
advance by the International Earth Rotation and Reference Systems Service (IERS), when needed to ensure that the difference between the UTC and UT1 readings will never exceed 0.9 seconds
On Thursday, May 17, 2018, Wes Turner firstname.lastname@example.org wrote:
AstroPy solves for leap seconds  according to the IAU ERFA (SOFA) library  and the IERS-B and IERS-A tables . IERS-B tables ship with AstroPy. The latest IERS-A tables ("from 1973 though one year into the future") auto-download on first use .
 http://docs.astropy.org/en/stable/time/#time-scales-for-time-deltas  http://docs.astropy.org/en/stable/time/#writing-a-custom-format  "Leap second day utc2tai interpolation" https://github.com/astropy/astropy/issues/5369  https://github.com/astropy/astropy/pull/4436  http://docs.astropy.org/en/stable/utils/iers.html
On Thursday, May 17, 2018, Alexander Belopolsky < email@example.com> wrote:
On Thu, May 17, 2018 at 3:13 PM Tim Peters firstname.lastname@example.org wrote:
Does that support the other way -- or do we never lose a leap second
(showing ignorance here)
Alexander covered the Python part of this, ...
No, I did not. I did not realize that the question was about skipping a second instead of inserting it. Yes, regardless of whether it is possible given the physics of Earth rotation, negative leap seconds can be supported. They simply become "gaps" in PEP 495 terminology. Check out PEP 495 and read "second" whenever you see "hour". :-)