System.Time.Clock Design Issues

[Bayley, Alistair]

> From: Ketil Malde [mailto:ketil+haskell at ii.uib.no] 
> 
> I don't think this is correct.  In UTC, extra leap seconds are denoted
> 23:59:60 and the missing ones have :59:58 as the last second.  Posix's
> seconds are thus not isomorphic to UTC.


Ahh, yes, sorry:
  http://cr.yp.to/proto/utctai.html
  http://tycho.usno.navy.mil/systime.html
  http://www.boulder.nist.gov/timefreq/pubs/bulletin/leapsecond.htm

I should read more...

So UTC and UT1 seem to be more-or-less equivalent, in that they both include
leap seconds. UT1 measures the actual earth day (so I assume it has some
fractional component for seconds), while UTC (which I assume counts integral
seconds) tracks UT1 with an error of +/-0.9s. UTC "differs from TAI by an
integral number of seconds".

TAI ignores leap seconds, but does track time accurately by always ticking
(i.e. not slowing down at a leap second). There are exactly 86400 seconds in
a TAI day, so the day will drift w.r.t. the earth's actual day, whereas UTC
tracks the actual day accurately by inserting leap seconds. So the TAI day
isn't very useful.

POSIX is "idealised" time, where days are always 23:59:59, and a second
sometimes takes two seconds.


Alistair.

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