Paul Eggert scripsit:
A count of UTC seconds since the Epoch is the same as a count of TAI seconds
Only if you are talking about broken-down labels for time. But I was talking about time expressed as a count of seconds. For example, the two adjacent real-time seconds with broken-down labels 1998-12-31 23:59:60 and 1999-01-01 00:00:00 UTC have the same count-of-seconds since the epoch.
No, I don't believe so. The two adjacent seconds you mention have the same _Posix_ time, but the number of elapsed UTC seconds = TAI seconds = SI seconds since the Epoch is not the same; ergo, on an Olsen-right system the values returned by gmtime will be different for these two labels.
Since TAI ticks along uniformly, and UTC-TAI jumps backwards by 1 second at the end of the inserted leap second, it logically follows that UTC also jumps backward by 1 second at the same moment.
Both TAI and UTC tick along uniformly at a rate of 1 SI second per second at all times (as opposed to other time scales which do not). It is simply that the broken-down time labels of UTC do something unusual at a leap second, whereas those of TAI do not. The statement that TAI-UTC changes applies to broken-down time labels, not to elapsed time. (I suspect that our disagreement is primarily or even entirely terminological.) -- "They tried to pierce your heart John Cowan with a Morgul-knife that remains in the http://www.ccil.org/~cowan wound. If they had succeeded, you would http://www.reutershealth.com become a wraith under the domination of the Dark Lord." --Gandalf