When a official time changes what's happend in real time ?

Paul Koning Paul_Koning at Dell.com
Tue Jun 2 10:53:10 UTC 2009

> Subject: When a official time changes what's happend in real time ?
> When a TZ change in France in the middle of the night i sleep so i
> never see if hour change in real time...
> I see on
> http://en.wikipedia.org/wiki/Network_Time_Protocol#Microsoft_Windows
> than W32TIME can have an 1-2 second error..
> is http://www.meinberg.de/english/sw/ntp.htm is more precise ?

That is a different subject, not really related.

The way to look at it goes like this:

1. Your computer keeps UTC.  Perhaps it has been told once and just
keeps ticking, or perhaps it is kept in sync with NTP.  In either case,
it keeps UTC with some accuracy, which depends on hardware and software.

2. The timezone (TZ setting) specifies an offset from UCT, and when --
if ever -- that offset changes.  For example, setting your TZ to France
specifies the offset is +1 hour in winter, +2 hours in summer.  When you
ask the computer to supply local time, it takes UTC plus the current
offset, that's the answer.

When the offset changes (on transition from summer time to winter time,
or vice versa), the offset simply changes.  That's not a time adjustment
in the way NTP does, it is simply a change to the offset variable.
Indeed, you do not need NTP, or the time adjusting machinery in the OS
that NTP requires, for TZ offset changes to work.

If you were watching the clock display (assuming the offset changes at 2
am local as it does in the USA), it would look like:  1:59:58, tick
1:59:59, tick 3:00:00, tick 3:00:01, and so on.  (Or 1:59:58, 1:59:59,
1:00:00, 1:00:01 ... for the other transition.)


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