[ntp:questions] Time going backwards

Richard B. Gilbert rgilbert88 at comcast.net
Thu Aug 14 03:03:59 UTC 2008

John Bartas wrote:
> Hi Folks,
> I have some questions about when (if ever) NTP will set time backward. 
> I'll be using it on Embedded Linux for time stamping video frames.
> I've found several mentions in RFC 1305 that this should not happen, and 
> also in section "Step Phase Adjustments" mention that it can jump 
> backwards.
> Is there a simple description anywhere if what variables affect this, 
> and how? I'd like to know when it's going to jump backwards on Linux - 
> what the rules are and what control I have over it.
> Any help would be appreciated.
> -JB-
> ..

I wouldn't get terribly upset about it.  Ntpd does not step the time 
unless it is grossly wrong.  Ntpd has been known to step *FORWARD* on 
certain Windows and MAYBE Linux systems when heavy disk activity 
resulted in missed clock ticks.  I can't think of anything likely to go 
wrong that would cause a *BACKWARD* step.

More information about the questions mailing list