So far there has not beeb a negative (backward) leap second and due to the problem of repeating a second there is a proposal to ban negative leap seconds in the future, just wait until you need a positive (forward) leap second.
On Wed, Oct 29, 2025 at 10:56 AM Paul Gilmartin < [email protected]> wrote: > On Wed, 29 Oct 2025 01:22:18 -0500, Mike Schwab <[email protected]> > wrote: > > >One feature is that some sites use different times in different regions. > >I. E. a Japanese, Indian, European, Eastern US, Western US time zones. > All > >implemented with different offsets from UTC. And IBM spreads leap seconds > >over the affected minute. > > ... > The practice is called "Leap Second Smearing". I can find no good > reference. > I hope that correction is made at midnight UTC everywhere, not local. > I understand that both Google and Amazon servers do it with different > smearing durations. > > It couldn't be done by TOD clock steering. > o I believe the maximum steering rate is a second in several hours, > not just a minute. > o It would contradict the statement in PlOps that TOD is kept at > TAI minus 10 seconds. > Is it done by repeated minuscule adjustments to CVTLSO? > > It was discussed here a few years ago that: > o Before a leap second all user processes are made nondispatchable. > o During the leap second 1 is added to CVtLSO. > o after the leap second user processes are dispatched. > > That leaves a hazard that if CVtLSO is accessed and > STCK is issued on opposite sides of the leap second, > in either order, there is a possibility of: > o Invalid UTC > o Duplicate UTC > o anachronistic UTC. > That can be avoided by such as: > Try: fetch CVTLSO > SGCK > compare CVTLSO to value previously fetched > BNE Try > Ugh. Two extra instructions executed a million times daily > to cover a pitfall that can occur at most twice a year, is > unlikely, and impossible to recreate for problem reporting. > > -- > gil > > ---------------------------------------------------------------------- > For IBM-MAIN subscribe / signoff / archive access instructions, > send email to [email protected] with the message: INFO IBM-MAIN > -- Mike A Schwab, Springfield IL USA Where do Forest Rangers go to get away from it all? ---------------------------------------------------------------------- For IBM-MAIN subscribe / signoff / archive access instructions, send email to [email protected] with the message: INFO IBM-MAIN
