[LEAPSECS] the big artillery

Zefram zefram at fysh.org
Mon Nov 3 05:35:44 EST 2014

Warner Losh wrote:
>TAI and UTC have a fixed offset relationship, it is true. However, UTC is
>computed in real time (with several varieties to choose from if you care
>about the nano-seconds), but TAI is a retrospective timescale that's not
>computed until after the fact.

These two notions conflict: a predetermined offset relationship means that
TAI and UTC can be trivially computed from each other, which means that
if one of them can be computed in real time then so can the other.  The
reality is that the difference between TAI and UTC is indeed predetermined
(and always integral seconds, post-1972), and *both* TAI and UTC are
paper time scales that are canonically only determined in retrospect.
Both have real-time realisations that differ by nanoseconds: each UTC(k)
realising UTC implies, via the well-known offset, a corresponding TAI(k)
realising TAI.  The notation "TAI(k)" isn't officially approved, but the
concept is perfectly meaningful, these time scales are readily accessible,
and they are de facto used in some applications.

There's a political shell game going on with some people trying to
suggest that there's a significant difference here between TAI and UTC,
trying to discourage the use of TAI by end users.  I can only guess at
the motivation: perhaps an attempt to keep some of the conceptual space
unsullied by the grubby mitts of actual applications.  Don't be fooled:
real-time TAI realisation is available to the masses.


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