On Sun, Oct 28, 2018 at 4:40 PM Mark H Weaver <m...@netris.org> wrote:
I believe you're making a subtle error in your identification of UTC > seconds with TAI seconds. And I in turn think you (or perhaps someone else)are making an error in your use of the term "UTC". > A TAI clock aims to measure the current TAI time, i.e. to measure the > time interval between the epoch and _you_ in units of TAI seconds, > i.e. SI seconds as observed on the geoid. > > In spatial terms, a TAI clock is analogous to a very precise measuring > tape which aims to measure the distance between you and an fixed > conventional point in space, analogous to the TAI epoch. > We agree so far. > In this analogy, a UTC clock is analogous to an equally precise > measuring tape that is _almost_ identical to the TAI analogue measuring > tape, but subtly different. If we place the two measuring tapes side by > side and ignore pre-1972, we see that the markings are in _precisely_ > the same positions along the entire length of the tape, without the > slightest deviation, even over long distances. > > The difference between the two measuring tapes is that they assign > different numbers to the markings, and moreover that the UTC analogue > has a small handful of places where two adjacent markings have the same > number assigned, and all subsequent numbers are shifted by 1. Now I think you are entirely right here, modulo a single term: what you are calling "UTC", I call (and I think correctly), "Posix". It is Posix time that has two identical adjacent markings. 126230400 |------------ > 126230400 |------------ > The numbers here are not UTC seconds since the Epoch, but Posix seconds since the Epoch. In short, there are exactly 86400 Posix seconds in a mean solar day, whereas in UTC reckoning there are exactly 1440 minutes, of which some may contain more (or less) than 60 seconds. That is why UTC clocks displayed 2016-12-13T23:59:60 during the last leap second. Computer clocks are normally Posix rather than UTC clocks, and the difference does not matter very often. > $1 = ((126230410 "1973-12-31T23:59:58Z") > (126230411 "1973-12-31T23:59:59Z") > (126230412 "1973-12-31T23:59:60Z") > (126230413 "1974-01-01T00:00:00Z") > (126230414 "1974-01-01T00:00:01Z")) > --8<---------------cut here---------------end--------------->8-- Good. > > If I understand correctly, 'time-utc->date' should never return a date > > object with 60 in the seconds field, because those extra seconds have no > > representation in time-utc. They only have representations in time-tai > > and time-monotonic. > Same story. Leap seconds have no representation in Posix time, but they do in UTC time. > As you can see, 1973-12-31T23:59:60Z and 1974-01-01T00:00:00Z have the > same representation in TIME-UTC. That is because what is called UTC time is in fact Posix time. -- John Cowan http://vrici.lojban.org/~cowan co...@ccil.org It was impossible to inveigle Georg Wilhelm Friedrich Hegel Into offering the slightest apology For his Phenomenology. --W. H. Auden, from "People" (1953)
