So internally, once we hit the leap second time, we step backwards.
Lovely waxing crescent moon last night. I could see it out of the north facing windows of our family room. It might be simpler in some sense to pretend that the moon (and the sun for that matter) always rises due east and set due west, but there it was, shining in the window because its declination last night happened to lie above the equator. Whatever time does, it does not step backwards. It may be simpler - in some sense - to build a clock or software interface that accommodates the lunar correction to the solar day by stepping backwards more or less contemporaneously with a leap second, but when you get down to it, this is - well - a naive kludge.
If you freeze time, then doing a time exchange during a leap second is going to give bogus results.
Time neither freezes...
If you step it back, then it will give good results, but there are other bad effects.
...nor steps back during a leap second. Just like the 29th of February, a leap second is an extra unit of time that is interpolated into the grand eternal sweep of history and causality. Why not design our computers, clocks and communications technology to implement that simple fact - just like our calendars and palm pilots recognize the fact of an extra day in February every four years? Hey gang! Let's put on a Gedankenexperimenten! Imagine we are faced with the prospect of a quadratically accelerating sequence of negative, rather than positive, leap seconds. (We'll ignore the trifling anthropic dilemma of the moon looming ever closer in our sky.) Two questions to ponder: How would we implement these? And would the ITU feel more driven - or rather less - to modify UTC as a result? To address the first question, we can start with the statutory requirement from ITU 460-4: "2.2 A positive leap-second begins at 23h 59m 60s and ends at 0h 0m 0s of the first day of the following month. In the case of a negative leap-second, 23h 59m 58s will be followed one second later by 0h 0m 0s of the first day of the following month" As we can see from the current discussion, system designers don't necessarily even try to implement statutory requirements. Notions about freezing time or of stepping it back arise - it seems to me - out of balking at the clear and explicit requirements of the task at hand. But in the case of a negative leap second, there would be no additional wrinkle in time to iron out or to double over - just the simple need to omit a second from the count. If we were merely faced with omitting a second every year or two, would the requirement really seem particularly onerous from the point of Posix or NTP or our other technology? Would GLONASS have noticed the event even at the minimal level seen this New Year? What is it about a 61s minute that is deemed more herculean than a 59s minute? We manage to deal with months containing 28, 29, 30, and 31 days. The great majority of months (4497/4800) aren't even denumerable with the length of our week. Why then is a requirement that one minute out of 800,000 accommodate one extra (or one fewer) second seen to be such an imposition? Especially when anybody who does find it so can simply choose to use TAI instead? Eppur si muove! Rob Seaman NOAO