[Dovecot] May 05 07:20:21 imap: Warning: Time jumped forwards 16 seconds

[Lorens Kockum]

On Thu, May 05, 2011 at 07:54:50PM +0100, Spyros Tsiolis wrote:
> 
> Seriously ?

Yes, Timo was (of course) both serious and correct.

ntpdate takes one or more NTP servers as parameters, and sets
your server's time to match that of the NTP servers. That may
well cause a jump, even a massive jump.

ntpd takes a list of NTP servers in its configuration file,
and uses them to make continual small adjustments. I seem to
remember that in some cases it is even capable of adjusting the
speed of your system clock according to its measurements. If the
difference is too great it will refuse to function and exit with
an error.

The usual way is to run ntpdate with -b option once at boot
(just after the network comes up and long before things like
dovecot and MTAs get started), and then start up ntpd.

The other way is to run ntpdate frequently, against an NTP
server you trust. It's not as good, but sometimes there may be
objections against running daemons, and if you're aiming at a
well-behaved NTP server the jumps should be minimal.

When running ntpd, the essential thing is to check that
it's actually doing its job. You do that with the command
"ntpdc". That will drop you to a prompt. The essential commands
are

	sysinfo
	peers
	server x.x.x.x
	sysinfo
	quit

sysinfo should give your stratum as somwhere between 3 and 5 (if
it's less you're probably doing something wrong, and if it's 16
you're not synchronized). peers should give one * sign in the
first column and some number of + signs.

After that overview, man ntpdate, man ntpd, and google :-)

HTH.