Saturday, May 16, 2015

NTP Sync & UTC

UTC (Universal Time Coordinated)


UTC (Universal Time Coordinated)) is an official standard for the current time and is independent from local time zones.

To convert UTC to local time, one would have to add or subtract the local time zone.


The following list will explain the main components of UTC:

o    Universal -  means that the time can be used everywhere in the world, meaning that it is independent from time zones (i.e. it's not local time).

o    Coordinated means that several institutions contribute their estimate of the current time, and UTC is built by combining these estimates.





UTC evolved from the former GMT (Greenwich Mean Time).

GMT was once used to set the clocks on ships before they left for a long journey. GMT had been adopted as the world's standard time.

Later GMT had been replaced as official standard time due to the fact that it was based on the mean solar time. And newer methods of time measurement showed that the mean solar time varied a lot by itself.



NTP (Network Time Protocol)


The internal clocks of most computers are rather inaccurate. To make every computer refer same time, either:

1.       You can correct the clock by hand on occasion, or

2.       You can use NTP to regulate your clock to be much more accurate.


This introductory article will try to show you how to use NTP to control and synchronize your system clock.


·         NTP provides accurate and synchronized time across the Internet.

·         NTP is an Internet protocol used to synchronize the clocks of computers to some time reference.

·         NTP is a way for computers to exchange information about the exact time, so that they can regulate their clocks.

·         NTP uses UTC as reference time

·         NTP is actually continuously running program that uses the Network Time Protocol to determine the correct time and regulate your computer's clock to show the correct time.

·         The NTP software can be obtained from It runs on Unix and Windows/NT systems

·         NTP can regulate your clock using a "reference clock", a device that obtains accurate time information from an outside source (Satellites) and delivers it to your computer.


Once you install the NTP software and start it running, you must provide it with a source of time information, either a reference clock, or with the addresses of other NTP daemons.

This document describes the critical steps of installing the software and configuring NTP to obtain time from other NTP daemons. Information about using a reference clock with NTP is at



Reference clocks:

NTP is organized in a hierarchical client-server model.

In the top of this hierarchy there are a small number of machines known as reference clocks.

A reference clock is known as stratum 0 and is typically a cesium clock or a Global Positioning System (GPS) that receives time from satellites. 


Why Time should be synchronized:

Time usually just advances. If you have communicating programs running on different computers, time still should even advance if you switch from one computer to another.

Obviously if one system is ahead of the others, the others are behind that particular one.


From the perspective of an external observer, switching between these systems would cause time to jump forward and back, a non-desirable effect.

For example - Just imagine some EMail message arrived five minutes before it was sent, and there even was a reply two minutes before the message was sent.


As a consequence, isolated networks may run their own wrong time, but as soon as you connect to the Internet, effects will be visible.

Thus time should be synchronized using NTP.






Arun Manglick