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 have 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.) paul