Editing Network Time Protocol (section)

== Leap seconds ==
On the day of a [[leap second]] event, ntpd receives notification from either a [[configuration file]], an attached reference clock, or a remote server. Although the NTP clock is actually halted during the event, because of the requirement that time must appear to be [[Monotonic function|strictly increasing]], any [[Process (computing)|processes]] that query the system time cause it to increase by a tiny amount, preserving the order of events. If a negative leap second should ever become necessary, it would be deleted with the sequence 23:59:58, 00:00:00, skipping 23:59:59.<ref name="7hQhm">{{cite web |url=http://www.eecis.udel.edu/~mills/leap.html |title=The NTP Timescale and Leap Seconds |author=David Mills |access-date=15 October 2013 |archive-url=https://web.archive.org/web/20130907021050/http://www.eecis.udel.edu/%7emills/leap.html |archive-date=7 September 2013 |url-status=live}}</ref>

An alternative implementation, called leap smearing, consists in introducing the leap second incrementally during a period of 24 hours, from noon to noon in UTC time. This implementation is used by Google (both internally and on their public NTP servers), Amazon AWS,<ref name="kYj2y">{{cite web |url=https://developers.google.com/time/smear |title=Google Developers Leap Smear |access-date=4 April 2019 |archive-url=https://web.archive.org/web/20190404122431/https://developers.google.com/time/smear |archive-date=4 April 2019 |url-status=live}}</ref> and Facebook.<ref>{{cite journal |last1=Obleukhov |first1=Oleg |title=Building a more accurate time service at Facebook scale |journal=Engineering at Meta |date=18 March 2020 |url=https://engineering.fb.com/2020/03/18/production-engineering/ntp-service/}}</ref> {{Proper name|chrony}} supports leap smear in {{mono|smoothtime}} and {{mono|leapsecmode}} configurations, but such use is not to be mixed with a public NTP pool as leap smear is non-standard and will throw off client calculation in a mix.<ref>{{cite web |title=chrony – Frequently Asked Questions |url=https://chrony.tuxfamily.org/faq.html#_should_be_a_leap_smear_enabled_on_ntp_server |website=chrony.tuxfamily.org}}</ref>