Editing Leap second (section)

==Problems==
[[File:ChronyControl screenshot.webp|thumb|upright=1.5|Screenshot of [[Chrony]]Control on [[macOS]], showing an insert second announcement by NTP on 30 June 2015.]]
===Time difference and sequence calculations===
To compute the elapsed time in seconds between two given UTC dates requires the consultation of a table of leap seconds, which needs to be updated whenever a new leap second is announced. Since leap seconds are known only 6 months in advance, time intervals for UTC dates further in the future cannot be computed.

===Missing announcements===
Although [[BIPM]] announces a leap second 6 months in advance, most time distribution systems ([[SNTP]], [[IRIG-B]], [[Precision Time Protocol|PTP]]) announce leap seconds at most 12 hours in advance,{{citation needed|date=October 2019}}<ref>{{Cite tech report |title=A Resilient Architecture for the Realization and Distribution of Coordinated Universal Time to Critical Infrastructure Systems in the United States |url=https://tsapps.nist.gov/publication/get_pdf.cfm?pub_id=933488 |date=November 2021 |doi=10.6028/NIST.TN.2187|doi-access=free }}</ref> sometimes only in the last minute and some even not at all ([[DNP3]]).{{citation needed|date=October 2019}}

===Implementation differences===
Not all clocks implement leap seconds in the same manner. Leap seconds in [[Unix time]] are commonly implemented by repeating 23:59:59 or adding the time-stamp 23:59:60. [[Network Time Protocol]] (NTP) freezes time during the leap second,<ref>{{Cite web |date=10 February 2010 |title=NIST Internet Time Service (ITS) |url=https://www.nist.gov/pml/time-and-frequency-division/time-distribution/internet-time-service-its |website=NIST}}</ref> some time servers declare "alarm condition".{{citation needed|date=October 2019}} Other schemes ''smear'' time in the vicinity of a leap second, spreading out the second of change over a longer period. This aims to avoid any negative effects of a substantial (by modern standards) step in time.<ref>{{citation|rfc=7164|author=Kevin Gross|title=RTP and Leap Seconds|date=March 2014}}</ref><ref name=google-smear /> This approach has led to differences between systems, as leap smear is not standardized and several different schemes are used in practice.<ref>{{cite web | website = Bureau International des Poids et Mesures | url = https://www.bipm.org/documents/20126/77823803/INTRODUCTION-DRAFT-RESOLUTION-D.pdf/62f31b32-bfe4-969f-5d29-1dc08cc907bb?version=1.2&t=1669373890355&download=true | title = Draft Resolution D: 'On the use and future development of UTC' | last1 = Dimarcq | first1 = Noël | last2 = Tavella | first2 = Patrizia  | date = 17 November 2022 | page = 7}}</ref>

===Textual representation===
The textual representation of a leap second is defined by BIPM as "23:59:60". There are programs that are not familiar with this format and may report an error when dealing with such input.

===Binary representation===
Most computer operating systems and most time distribution systems represent time with a binary counter indicating the number of seconds elapsed since an arbitrary [[epoch]]; for instance, since {{nowrap|1970-01-01}} 00:00:00 in POSIX machines or since {{nowrap|1900-01-01}} 00:00:00 in NTP. This counter does not count positive leap seconds, and has no indicator that a leap second has been inserted, therefore two seconds in sequence will have the same counter value. Some computer operating systems, in particular Linux, assign to the leap second the counter value of the preceding, 23:59:59 second ({{nowrap|59–59–0}} sequence), while other computers (and the IRIG-B time distribution) assign to the leap second the counter value of the next, 00:00:00 second ({{nowrap|59–0–0}} sequence).{{citation needed|date=October 2019}} Since there is no standard governing this sequence, the timestamp of values sampled at exactly the same time can vary by one second. This may explain flaws in time-critical systems that rely on timestamped values.<ref>{{Cite journal |last1=Benzler |first1=Justus |last2=Clark |first2=Samuel J. |date=30 March 2005 |title=Toward a Unified Timestamp with explicit precision |journal=[[Demographic Research (journal)|Demographic Research]] |volume=12 |issue=6 |pages=107–140 |doi=10.4054/DemRes.2005.12.6 |issn=1435-9871 |pmc=2854819 |pmid=20396403}}</ref>

===Others===
Several models of global navigation satellite receivers have software flaws associated with leap seconds:

* Some older versions of Motorola Oncore VP, UT, GT, and M12 GPS receivers had a software bug that would cause a single timestamp to be off by a day if no leap second was scheduled for 256 weeks. On 28 November 2003, this happened. At midnight, the receivers with this firmware reported 29 November 2003, for one second and then reverted to 28 November 2003.<ref>{{cite web|url=http://www.leapsecond.com/notes/leapsec256.htm|title=256-Week Leap Second Bug|date=2 July 2013|url-status=live|archive-url=https://web.archive.org/web/20160304002759/http://www.leapsecond.com/notes/leapsec256.htm|archive-date=4 March 2016}}</ref><ref>{{cite web|url=http://compgroups.net/comp.protocols.time.ntp/motorola-oncore-receivers-and-leap-se/287130|title=Motorola Oncore receivers and Leap Second bug|date=2 July 2013|url-status=usurped|archive-url=https://web.archive.org/web/20130118233907/http://compgroups.net/comp.protocols.time.ntp/motorola-oncore-receivers-and-leap-se/287130|archive-date=18 January 2013}}</ref>
* Older Trimble GPS receivers had a software flaw that would insert a leap second immediately after the [[List of GPS satellites|GPS constellation]] started broadcasting the next leap second insertion time (some months in advance of the actual leap second), rather than waiting for the next leap second to happen. This left the receiver's time off by a second in the interim.<ref>{{cite web|url=http://www.guralp.com/howtos/leap-second-problem-with-older-gps-receivers.shtml|title=Leap-second problem with older GPS receivers|date=19 November 2014|url-status=live|archive-url=https://web.archive.org/web/20141129055128/http://www.guralp.com/howtos/leap-second-problem-with-older-gps-receivers.shtml|archive-date=29 November 2014}}</ref><ref>{{cite web|url=http://www.spirent.com/Blogs/Positioning/2015/May/How_Leap_Seconds_Can_Interfere_with_GNSS_Receivers|title=How Leap Seconds Can Interfere with GNSS Receivers|date=13 May 2015|url-status=live|archive-url=https://web.archive.org/web/20160306014132/http://www.spirent.com/Blogs/Positioning/2015/May/How_Leap_Seconds_Can_Interfere_with_GNSS_Receivers|archive-date=6 March 2016}}</ref>
* Older Datum/Symmetricom TymServe 2100 GPS receivers apply a leap second as soon as the a leap second notification is received, instead of waiting for the correct date. The manufacturers no longer supports these models and no corrected software is available. A workaround has been described and tested, but if the GPS system rebroadcasts the announcement, or the unit is powered off, the problem will occur again.<ref>{{cite mailing list|url=http://permalink.gmane.org/gmane.comp.time.nuts/43942|title=Symmetricom TymServe 2100-GPS currently fails with GPS offset|mailing-list=time-nuts|url-status=live|archive-url=https://web.archive.org/web/20150217202809/http://permalink.gmane.org/gmane.comp.time.nuts/43942|archive-date=17 February 2015}}</ref>
* Four different brands of navigational receivers that use data from [[BeiDou]] satellites were found to implement leap seconds one day early.<ref>{{cite web |date=3 March 2015 |title=BeiDou Numbering Presents Leap-Second Issue |url=https://www.gpsworld.com/beidou-numbering-presents-leap-second-issue/ |publisher=GPS World}}</ref>  This was traced to a bug related to how the BeiDou protocol numbers the days of the week.

Several software vendors have distributed software that has not properly functioned with the concept of leap seconds:

* NTP specifies a flag to inform the receiver that a leap second is imminent. However, some NTP server implementations have failed to set their leap second flag correctly.<ref>{{cite web |url=http://www.maths.tcd.ie/~dwmalone/time/leaps/ |title=NTP Leap Bits |first1=David |last1=Malone |access-date=1 December 2019 |url-status=live |archive-url=https://web.archive.org/web/20140722122446/http://www.maths.tcd.ie/~dwmalone/time/leaps/ |archive-date=22 July 2014}}</ref><ref>{{cite conference |first1=David |last1=Malone |year=2016 |title=The Leap Second Behaviour of NTP Servers |book-title=Proc.Traffic Monitoring and Analysis workshop |url=http://tma.ifip.org/2016/papers/tma2016-final27.pdf |access-date=23 October 2016 |url-status=live |archive-url=https://web.archive.org/web/20161023201948/http://tma.ifip.org/2016/papers/tma2016-final27.pdf |archive-date=23 October 2016}}</ref><ref name="sync_Leap2016">{{cite conference |last1=Cao |first1=Yi |last2=Veitch |first2=Darryl |title=Network Timing, Weathering the 2016 Leap Second |doi=10.1109/INFOCOM.2018.8486286 |pages=1826–1834 |conference=IEEE Infocom 2018 |location=Honolulu, Hawaii |date= 15–19 April 2018|hdl=10453/130538 |hdl-access=free }}</ref><ref name="sync_Leap2015">{{cite conference |last1=Veitch |first1=Darryl |last2=Vijayalayan |first2=Kanthaiah |title=Network Timing and the 2015 Leap Second |doi=10.1007/978-3-319-30505-9_29 |pages=385–396 |book-title=Proc. of PAM 2016 |location=Heraklion, Crete, Greece |date=1 April 2016|hdl=10453/43923 |hdl-access=free }}</ref> Some NTP servers have responded with the wrong time for up to a day after a leap second insertion.<ref>{{cite web|url=http://www.satsignal.eu/ntp/ntp-events.htm|title=NTP Events|work=satsignal.eu|url-status=live|archive-url=https://web.archive.org/web/20140718204646/http://www.satsignal.eu/ntp/ntp-events.htm|archive-date=18 July 2014}}</ref>
* A number of organizations reported problems caused by flawed software following the leap second that occurred on 30 June 2012. Among the sites which reported problems were [[Reddit]] ([[Apache Cassandra]]), [[Mozilla]] ([[Hadoop]]),<ref>{{cite magazine |date=1 July 2012 |title='Leap Second' Bug Wreaks Havoc Across Web |url=https://www.wired.com/wiredenterprise/2012/07/leap-second-bug-wreaks-havoc-with-java-linux/ |url-status=live |magazine=Wired |archive-url=https://web.archive.org/web/20140328124116/http://www.wired.com/wiredenterprise/2012/07/leap-second-bug-wreaks-havoc-with-java-linux |archive-date=28 March 2014}}</ref> [[Qantas]],<ref>{{cite web |date=1 July 2012 |title=Leap second crashes Qantas and leaves passengers stranded |url=http://www.news.com.au/travel/news/leap-second-crashes-qantas-and-leaves-passengers-stranded/story-e6frfq80-1226413961235 |url-status=live |archive-url=https://web.archive.org/web/20120701162924/http://www.news.com.au/travel/news/leap-second-crashes-qantas-and-leaves-passengers-stranded/story-e6frfq80-1226413961235 |archive-date=1 July 2012 |publisher=News Limited}}</ref> and various sites running Linux.<ref>{{cite web |title=Anyone else experiencing high rates of Linux server crashes during a leap second day? |url=http://serverfault.com/questions/403732/anyone-else-experiencing-high-rates-of-linux-server-crashes-during-a-leap-second |url-status=live |archive-url=https://web.archive.org/web/20120709110353/http://serverfault.com/questions/403732/anyone-else-experiencing-high-rates-of-linux-server-crashes-during-a-leap-second |archive-date=9 July 2012 |publisher=Serverfault.com}}</ref>
* Despite the publicity given to the 2015 leap second, a small number of network failures occurred due to leap second-related software errors of some routers.<ref>{{cite news |last1=Shulman |first1=Eden |title=Beta Boston |newspaper=Boston Globe |url=http://www.betaboston.com/news/2015/07/02/leap-second-confuses-some-internet-routers-bringing-down-a-small-slice-of-networks/ |url-status=live |access-date=27 September 2015 |archive-url=https://web.archive.org/web/20150929011652/http://www.betaboston.com/news/2015/07/02/leap-second-confuses-some-internet-routers-bringing-down-a-small-slice-of-networks/ |archive-date=29 September 2015}}</ref>  Several older versions of the [[Cisco Systems]] Nexus 5000 Series Operating System NX-OS (versions 5.0, 5.1, 5.2) are affected by leap second bugs.<ref>{{cite web |date=24 July 2015 |title=Cisco Bug: CSCub38654 – N5K: Switch hang/lock up may occur due to Leap second update |url=https://quickview.cloudapps.cisco.com/quickview/bug/CSCub38654 |url-status=live |archive-url=https://web.archive.org/web/20160308010403/https://quickview.cloudapps.cisco.com/quickview/bug/CSCub38654 |archive-date=8 March 2016 |publisher=Cisco}}</ref>

Some businesses and service providers have been impacted by leap-second related software bugs:

* In 2015, interruptions occurred with [[Twitter]], [[Instagram]], [[Pinterest]], [[Netflix]], [[Amazon (company)|Amazon]], and [[Apple Inc|Apple's]] music streaming series [[Beats 1]].<ref>{{cite news |author=Sarah Knapton |date=1 July 2015 |title=Leap Second confuses Twitter and Android |newspaper=[[The Daily Telegraph]] |url=https://www.telegraph.co.uk/news/science/science-news/11710148/Leap-Second-confuses-Twitter-and-Android.html |url-status=live |archive-url=https://web.archive.org/web/20151006002636/http://www.telegraph.co.uk/news/science/science-news/11710148/Leap-Second-confuses-Twitter-and-Android.html |archive-date=6 October 2015}}</ref>
* Leap second software bugs in Linux reportedly affected the [[Amadeus Altéa]] airlines reservation system, used by Qantas and [[Virgin Australia]], in 2015.<ref>{{Cite web |last=Clarke |first=Gavin |date=8 August 2016 |title=Power cut crashes Delta's worldwide flight update systems |url=https://www.theregister.co.uk/2016/08/08/computer_fault_takes_down_delta/ |url-status=live |archive-url=https://web.archive.org/web/20170104002416/http://www.theregister.co.uk/2016/08/08/computer_fault_takes_down_delta/ |archive-date=4 January 2017 |access-date=3 January 2017 |website=[[The Register]]}}</ref>
* [[Cloudflare]] was affected by a leap second software bug. Its [[DNS]] resolver implementation incorrectly calculated a negative number when subtracting two timestamps obtained from the [[Go programming language]]'s <code>time.Now()</code>function, which then used only a [[real-time clock]] source.<ref>{{cite web |date=1 January 2017 |title=How and why the leap second affected Cloudflare DNS |url=https://blog.cloudflare.com/how-and-why-the-leap-second-affected-cloudflare-dns/ |url-status=live |archive-url=https://web.archive.org/web/20170102112428/https://blog.cloudflare.com/how-and-why-the-leap-second-affected-cloudflare-dns/ |archive-date=2 January 2017 |publisher=Cloudflare}}</ref>  This could have been avoided by using a monotonic clock source, which has since been added to Go 1.9.<ref>{{cite web |title=#12914 runtime: time: expose monotonic clock source |url=https://github.com/golang/go/issues/12914 |url-status=live |archive-url=https://web.archive.org/web/20170320012113/https://github.com/golang/go/issues/12914 |archive-date=20 March 2017 |access-date=5 January 2017 |website=GitHub}}</ref>
* The [[Intercontinental Exchange]], parent body to 7 clearing houses and 11 stock exchanges including the [[New York Stock Exchange]], chose to cease operations for 61 minutes at the time of the 30 June 2015, leap second.<ref>{{cite web|url=https://www.theice.com/leap-second|title=ICE Market Update – Leap Second Impact|publisher=[[Intercontinental Exchange]]|url-status=dead|archive-url=https://web.archive.org/web/20150505061854/https://www.theice.com/leap-second|archive-date=5 May 2015}}</ref>

There were misplaced concerns that farming equipment using GPS navigation during harvests occurring on 31 December 2016, would be affected by the 2016 leap second.<ref>{{Cite news|url=http://www.abc.net.au/news/2016-12-30/2016-leap-second-correction-for-gps-farmers/8154156|title=Got a second – the world time needs it|date=30 December 2016|newspaper=ABC Rural|language=en-AU|access-date=3 January 2017|url-status=live|archive-url=https://web.archive.org/web/20170102192801/http://www.abc.net.au/news/2016-12-30/2016-leap-second-correction-for-gps-farmers/8154156|archive-date=2 January 2017}}</ref> GPS navigation makes use of [[GPS time]], which is not impacted by the leap second.<ref>{{citation|url=https://www.scientificamerican.com/article/how-fast-is-the-earth-mov/|date=26 October 1998|title=How fast is the earth moving? Rhett Herman, a physics professor at Radford University in Virginia, supplies the following answer|publisher=[[Scientific American]]}}</ref>

Due to a software error, the UTC time broadcast by the NavStar GPS system was incorrect by about 13 microseconds on 25–26 January 2016.<ref>{{cite news|url=https://elpromatime.com/wp-content/uploads/2022/08/US_AirForce_OfficialPressRelease.pdf |title=Air Force Official Press Release – GPS Ground System Anomaly}}</ref><ref>{{cite news |url=https://tf.nist.gov/general/pdf/2886.pdf |title=The effects of the January 2016 UTC offset anomaly on GPS-controlled clocks monitored at NIST |first1=Jian |last1=Yao |first2=Michael A. |last2=Lombardi |first3=Andrew N. |last3=Novick |first4=Bijunath |last4=Patla |first5=Jeff A. |last5=Sherman |first6=Victor |last6=Zhang}}</ref>