On Thu 2005/08/04 23:25:03 -0400, "John.Cowan" wrote
in a message to: LEAPSECS_at_ROM.USNO.NAVY.MIL
>Posix time is not a count of seconds, it is not aligned with TAI, and it is
not
>similar to TAI. TAI is truly a count of SI seconds. Posix time assigns
>a numeric label to each SI second, but some consecutive pairs of seconds
>are assigned the same label (namely, a leap second and the second before it).
You are saying that POSIX time, unix system time, is UTC, right?
http://en.wikipedia.org/wiki/Unix_time supports that but apparently
POSIX time keeping is a bit of a shambles - check Steve Allen's web page
http://www.ucolick.org/~sla/leapsecs/onlinebib.html or the man page for
time(2). (It also begs the question - why does unix maintain a table of
leap seconds?)
Whatever, I would guess that most computers that care about timekeeping
these days have their clocks adjusted via NTP - sound plausible?
NTP jumps through hoops to force unix system clocks to skip a beat when
a leap second is inserted (see the wikipedia URL). How much simpler
would it be if unix system clocks maintained TAI, with NTP distributing
TAI together with a table of leap seconds for conversion to civil time?
The remaining change would be to modify gettimeofday() and friends to
apply that table.
Apparently such "complexity in the library functions" was what swayed
the POSIX committee against using TAI in the first place. On the bright
side, the wikipedia URL contains the following statement:
As of 2004, POSIX has new interfaces making several different time
scales available to programs, splitting up the many uses to which
Unix times have traditionally been put. The future is one where time
values are accompanied by explicit labels of the time scale defining
their significance. Unix time as described in this article will
still be in wide use for decades to come, but is likely to be
increasingly treated as a legacy system and superseded by better-
defined systems.
An optimistic interpretation of this would be that POSIX.1 will provide
for unix time to be maintained as TAI.
A pessimistic scenario would be that finally when POSIX caught up with
leap seconds they were dropped!
Mark Calabretta
ATNF
Received on Fri Aug 05 2005 - 00:53:30 PDT