Magnus Danielson magnus at rubidium.dyndns.org
Sat Oct 10 11:10:17 EDT 2009


Joe Gwinn wrote:

> At 3:28 PM +0200 10/10/09, Magnus Danielson wrote:

>> M. Warner Losh wrote:

>>> In message: <4ACFF759.3090903 at rubidium.dyndns.org>

>>> Magnus Danielson <magnus at rubidium.dyndns.org> writes:

>>> : M. Warner Losh wrote:

>>> : > In message:

>>> <13205C286662DE4387D9AF3AC30EF456AFA8697A05 at EMBX01-WF.jnpr.net>

>>> : > Jonathan Natale <jnatale at juniper.net> writes:

>>> : > : AFAIK, routers also just re-sych. The OS's are not capable of

>>> : > : xx:xx:60 time. For reading router logs this is fine in most cases

>>> : > : which is all NTP is really for. I don't think they simply step

>>> the

>>> : > : time, I am pretty sure they do tweak the freq. I could be

>>> wrong and

>>> : > : I am NOT representing Juniper here, just my thoughts. :-)

>>> : > : > FreeBSD will cope with the xx:xx:60 second correctly,

>>> assuming it is

>>> : > told about the leapsecond soon enough. Not all other parts of the

>>> : > system can cope with the xx:xx:60, but that's a posix time_t

>>> : > limitation that you can't do anything about[*].

>>> : > : > Warner

>>> : > : > [*] The 'right' timezone files attempt to do things

>>> correctly, but in

>>> : > doing so they break time_t definition...

>>> : : I assumed you meant to say that it breaks the POSIX time_t

>>> definition.


>>> Yes. The most current time_t definition is the one codified by POSIX.

>>> Older standards are fuzzier about what time_t really means.


>> Indeed. As there exist several time_t definitions, I wanted to make

>> sure you was refering to the POSIX mapping of UTC time into time_t,

>> which forms an "interesting" timescale of its own, almost but not

>> close enough to UTC.


> By definition, POSIX Time is closer to TAI than to UTC, but in practice

> time in POSIX-compliant computers is usually NTP steered to approximate

> UTC (most common) or to GPS System Time (where leapseconds cannot be

> tolerated).

As the text of subclause 4.14 of the POSIX base standard defines it, it
is based on "Coordinated Universal Time" and the "name" is mapped into
seconds as defined by the mapping function. This makes it follow UTC
while maintaining the mental feel of being TAI-based without any leap
seconds, but it is closer to UTC as only occasionally (on the leap
second second) differs by a second during a second while it has a so far
constantly increasing difference to TAI. So on average it is much closer
to UTC than TAI.

So I respectfully disagree with your statement that POSIX Time is closer
to TAI than UTC. I think that it is closer to UTC and that the NTP
steering honour the POSIX UTC to time_t mapping.

A user wishing to display correct UTC time during leap-second would need
to querry the NTP kernel over the provided interface to recover the
extra information, which is possible when the NTP has the necessary
leapsecond information and is enabled.

I had the distinct memory that we discussed this indepth some time ago
both on and off list(s).


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