section_path: Definition/Leap seconds/Variant that counts leap seconds
section_name: Variant that counts leap seconds
paragraph_text: 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; some Linux systems are configured this way. 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).
  
  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 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 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 TAI rather than 1970-01-01T00:00:10 TAI, was proposed for inclusion in ISO C's time.h, but only the UTC part was accepted in 2011. A tai_clock does, however, exist in C++20.