Editing Unix time (section)

==== Variant that counts leap seconds ====
Another, much rarer, non-conforming variant of Unix time keeping involves incrementing the value for all seconds, including leap seconds;<ref>{{cite web|url=https://www.iana.org/time-zones/repository/tz-link.html#precision|work=Sources for time zone and daylight saving time data|title=Precision timekeeping|quote=The tz code and data support leap seconds via an optional "right" configuration where a computer's internal time_t integer clock counts every TAI second, as opposed to the default "posix" configuration where the internal clock ignores leap seconds. The two configurations agree for timestamps starting with 1972-01-01 00:00:00 UTC (time_t 63 072 000) and diverge for timestamps starting with time_t 78 796 800, which corresponds to the first leap second 1972-06-30 23:59:60 UTC in the "right" configuration, and to 1972-07-01 00:00:00 UTC in the "posix" configuration.|access-date=30 May 2022|archive-date=16 October 2017|archive-url=https://web.archive.org/web/20171016230241/http://www.iana.org/time-zones/repository/tz-link.html#precision|url-status=live}}</ref> some Linux systems are configured this way.<ref name="ntp-time-scales">{{cite web|title=Time Scales|url=https://support.ntp.org/bin/view/Support/TimeScales|work=Network Time Protocol Wiki|date=24 July 2019|access-date=12 January 2020|archive-date=12 January 2020|archive-url=https://web.archive.org/web/20200112103701/https://support.ntp.org/bin/view/Support/TimeScales|url-status=live}}</ref> Time kept in this fashion is sometimes referred to as "TAI" (although timestamps can be converted to UTC if the value corresponds to a time when the difference between TAI and UTC is known), as opposed to "UTC" (although not all UTC time values have a unique reference in systems that do not count leap seconds).<ref name="ntp-time-scales" />

Because TAI has no leap seconds, and every TAI day is exactly 86400 seconds long, this encoding is actually a pure linear count of seconds elapsed since 1970-01-01T00:00:10{{nbsp}}TAI. This makes time interval arithmetic much easier. Time values from these systems do not suffer the ambiguity that strictly conforming POSIX systems or NTP-driven systems have.

In these systems it is necessary to consult a table of leap seconds to correctly convert between UTC and the pseudo-Unix-time representation. This resembles the manner in which time zone tables must be consulted to convert to and from [[civil time]]; the [[IANA time zone database]] includes leap second information, and the sample code available from the same source uses that information to convert between TAI-based timestamps and local time. Conversion also runs into definitional problems prior to the 1972 commencement of the current form of UTC (see section [[#UTC basis|UTC basis]] below).

This system, despite its superficial resemblance, is not Unix time. It encodes times with values that differ by several seconds from the POSIX time values. A version of this system, in which the epoch was 1970-01-01T00:00:00{{nbsp}}TAI rather than 1970-01-01T00:00:10{{nbsp}}TAI, was proposed for inclusion in ISO C's [[C date and time functions|{{code|time.h}}]], but only the UTC part was accepted in 2011.<ref>{{cite web |author=Markus Kuhn |title=Modernized API for ISO C |url=https://www.cl.cam.ac.uk/~mgk25/time/c/ |website=www.cl.cam.ac.uk |access-date=31 August 2020 |archive-date=26 September 2020 |archive-url=https://web.archive.org/web/20200926224028/https://www.cl.cam.ac.uk/~mgk25/time/c/ |url-status=live }}</ref> A {{code|tai_clock}} does, however, exist in C++20.