[ntp:questions] Number of Stratum 1 & Stratum 2 Peers
nomail at example.com
Wed Dec 17 09:36:00 UTC 2014
Martin Burnicki <martin.burnicki at meinberg.de> wrote:
> The main problem is that the underlying system time (often POSIX, which
> just counts seconds since an epoch) has the *same* time stamp art the
> beginning and end of the leap second.
> In order to do the conversion correctly you need to know if the current
> second is the leap second, or not, i.e. you need some status flag in
> addition to the raw (e.g. POSIX) timestamp.
> This is basically similar to what you have at the end of DST, where (in
> local time) a whole hour is passed twice. You need to know the DST
> status to determine unambiguously if it's the first or the second turn.
Yes, but in situations where you care about that you store just the
system time, and the conversion to local time can be repeated later.
Thanks to the powerful conversion routines in glibc this can be done
even way in the past (because it remembers DST rules from previous years).
Unfortunately, the same mechanism isn't used for leap seconds.
There would be no problem at all when the system time ticked in TAI
and the addition of the leap seconds is done via some rule table similar
to the local time rules. ntpd would not even be involved in the problem.
> A main reason for the problems with leap seconds is that POSIX has just
> ignored them when they defined their standards on timekeeping.
That is the problem. And probably also the decision in Unix to count
UTC seconds since Jan 1st, 1970 (instead of TAI seconds).
Back then it did not matter that much in a timesharing system. The
system clock was mainly a convenience feature, not critical to anything.
(it usually had to be set on every boot and this was done by typing
the time seen on the operator's wristwatch, hence the term "wristwatch time")
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