John Cowan <jcowan@reutershealth.com> writes:
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. This is according to the official definition of UTC. Here's an example of the official wording: The difference between UTC and the International Atomic Time TAI is: from 1997 July 1, 0h UTC, to 1999 January 1, 0h UTC : UTC-TAI = - 31s from 1999 January 1, 0h UTC, until further notice : UTC-TAI = - 32s This is a quote from IERS Bulletin C 16 (1998-07-17) <http://hpiers.obspm.fr/eoppc/bul/bulc/bulletinc.16>. 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.
POSIX time counts the number of non-leap seconds since the Epoch
Yes. More precisely, POSIX time is defined by the following C-language expression, for years>=1970 (i.e., tm_year >= 70): tm_sec + tm_min*60 + tm_hour*3600 + tm_yday*86400 + (tm_year-70)*31536000 + ((tm_year-69)/4)*86400 - ((tm_year-1)/100)*86400 + ((tm_year+299)/400)*86400 Reference: <http://www.opengroup.org/onlinepubs/000095399/basedefs/xbd_chap04.html#tag_0...> This equation says that the POSIX clock hops backward by 1 second immediately after an inserted leap second (i.e., a second where tm_sec == 60). This is consistent with the official definition of UTC quoted above.