# [ntp:questions] NTPD can take 10 hours to achieve stability

Chris Albertson albertson.chris at gmail.com
Mon Apr 18 04:36:14 UTC 2011

```On Sun, Apr 17, 2011 at 7:58 PM, C BlacK <rblak at non.net> wrote:
> Why would it take ntpd ten hours to achieve its accuracy?  Can this be explained in laymans terms and
> mathematically

In very simple terms....

Ignore NTP and computers and think about an old mechanical clock.
Lets say you wanted to adjust the rate of the clock to one part in
10,000.  But the hands can only be read to the nearest one second but
you are lucky to have an atomic clock nearby.  In theory it would take
you 10,000 seconds to know if you adjusted the clock correctly.  In
other words it would take 10,000 seconds for the clock to gain or
loose one second relative to your standard.

NTP works kind of like that.  It uses a set of reference clocks and
watches the rate of your local clock relative to the reference
clock(s) and depending on details it make time some time

--
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Chris Albertson
Redondo Beach, California

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