[LEAPSECS] POSIX Time (was WP7A)

[Joe Gwinn]

Magnus,

At 5:10 PM +0200 10/10/09, Magnus Danielson wrote:

>Joe,

>

>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.


Be careful to distinguish "broken down time" (an ascii string that 
resembles UTC) from the underlying clock (two 32-bit integers that 
are supposed to count SI seconds and nanoseconds since the POSIX 
Epoch).

If you read the POSIX standard, you will find that the length of the 
day is defined as exactly 86,400 seconds, no more no less.  So, leap 
seconds are by definition forbidden.  This was the intent of the 
committee.

Another common source of confusion is that the POSIX Epoch is an 
instant defined in UTC terms, but one can define an instant in any 
timescale one wishes, and the instant may appear in all of them but 
belongs to none of them.

However, most people who care even a little about time use NTP to 
synchronize their computer's time to something, and by far the bulk 
of the relevant timeservers publish UTC, never mind the formal 
definition of POSIX Time.  So, in practice time on such systems is 
neither fish nor fowl, being neither TAI not UTC, instead being a 
mixture.



>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.


A tall order for sure, and one is completely at the mercy of the 
operating system kernel developers, who may be hazy on the details of 
time.

In the systems I have had a hand in, the computer clocks are 
synchronized to GPS System Time (because those systems cannot 
tolerate the discontinuities caused by leap seconds) and UTC is made 
by the application software only where needed when needed.



>I had the distinct memory that we discussed this in depth some time 

>ago both on and off list(s).


You've let our secret out!  Yes, it was in January 2009.

Joe Gwinn