Warner Losh wrote:
The IERS bulletin C is a little different than the ITU TF.460:
Leap seconds can be introduced in UTC at the end of the months of December
or June, depending on the evolution of UT1-TAI. Bulletin C is mailed every
six months, either to announce a time step in UTC, or to confirm that there
will be no time step at the next possible date.
Unfortunately, these IERS bulletins are dreadfully badly worded and
seem to assume current practice rather than fully defining what they
mean. E.g., Bulletin C 32, dated 19 July 2006
NO positive leap second will be introduced at the end of December 2006.
So we still don't know officially if there was a negative leap second
then and we still don't officially know if there will be a leap second
at the end of this month.
UTC is defined by the CCIR Recommendation 460-4 (1986). It differs
from TAI by an integral number of seconds, in such a way that UT1-UTC stays
smaller than 0.9s in absolute value. The decision to introduce a leap second
in UTC to meet this condition is the responsability of the IERS. According to
the CCIR Recommendation, first preference is given to the opportunities at the
end of December and June,and second preference to those at the end of March
and September. Since the system was introduced in 1972 only dates in June and
December have been used.
Again, this is the truth but not the whole truth as it doesn't mention
the third preference opportunities at the ends of other months - but
it'll be a while until those are needed.
(Also, they can't spell "responsibility" :-)