[ntp:questions] Quick sync between two computers not connected to the internet

Richard B. Gilbert rgilbert88 at comcast.net
Sat Mar 20 17:06:38 UTC 2010

cnoyes2 wrote:
> I'm trying to sync the time on 2 Windows XP computers that are not on
> the internet and never will be. For testing purposes, they need to
> have the same time. The time does not need to be accurate. What's the
> simplest way to sync these computers?
> I've done a lot of searching, but can't seem to find much about
> computers not attached to the internet.
> I've tried net time \\computer_ip /set /yes, but I get an access
> denied error.
> Thanks in advance.

How close do the two clocks have to be?  It's fairly easy to get two 
computers to agree within five or ten seconds with no special equipment 
or software required.  If you need to be within five or ten milliseconds 
it starts getting difficult and possibly expensive.  If you need to be 
within five or ten microseconds it can still be done but it will take 
some fairly expensive equipment.

The easiest thing to do if you can site an antenna where it will have an 
  unobstructed view of the sky, is to purchase a "hardware reference 
clock".  The cheapest goes for about $100 US.  It's called a GPS18-LVC 
if memory serves me.  If my memory fails me I'm sure someone will be 
delighted to point it out!  ;-)

The GPS18-LVC and other similar devices, get time from the NAVSTAR (GPS) 
satellites.  The antenna is generally smaller than a hockey puck.  I 
have mine on top of a "Leaf Guard" rain gutter.  They have a Pulse Per 
Second (PPS) output.  One edge of the pulse is generally within 50 
nanoseconds of the true time.  There is also an RS232C serial output 
that tells you which second the pulse is marking.

NTPD has drivers for a number of different devices.  In addition to GPS 
satellites there are VLF and HF broadcasts provided by the National 
Institute of Standards and Technology (NIST) in the US.  Other countries 
also provide radio time signals.

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