How to Correct Server Time

The method you choose for correcting the time on your server depends on how far off the server's clock is. If the difference is small, use the first method. If the clock is significantly behind or ahead, it's better not to adjust it in a single step — it's safer to change the time gradually.

Configuration on Ubuntu/Debian

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Quick Fix

To quickly change the time on the cloud server, use the ntpdate utility. You need sudo privileges to install it:

apt-get install ntpdate

To update the time once:

/usr/sbin/ntpdate 1.north-america.pool.ntp.org

Here, the NTP pool is the address of a trusted server used to synchronize the time. For the USA, you can use NTP servers from this page. You can find pool zones for other regions at ntppool.org.

You can also set up automatic time checks using cron:

crontab -e
00 1 * * * /usr/sbin/ntpdate 1.north-america.pool.ntp.org

This schedules synchronization once a day.

Instead of a set interval, you can specify a condition. For example, to synchronize the time on every server reboot using cron reboot:

crontab -e
@reboot /usr/sbin/ntpdate 1.north-america.pool.ntp.org

Gradual Correction

To update the time gradually, install the ntp utility on Ubuntu or Debian. It works as follows:

• The utility checks data from synchronization servers defined in the configuration.
• It calculates the difference between the current system time and the reference time.
• NTP gradually adjusts the system clock. This gradual correction helps avoid issues in other services caused by sudden time jumps.

Install NTP:

apt-get install ntp

For the utility to work correctly, configure it in the file /etc/ntp.conf. Add NTP servers like:

server 0.north-america.pool.ntp.org
server 1.north-america.pool.ntp.org iburst
server 2.north-america.pool.ntp.org
server 3.north-america.pool.ntp.org

The iburst option improves accuracy by sending multiple packets at once instead of just one.

You can also set a preferred data source using the prefer option:

server 0.ubuntu.pool.ntp.org iburst prefer

After each configuration change, restart the utility:

/etc/init.d/ntp restart

Configuration on CentOS

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The method choice rules are the same. If you need to correct a difference of a few seconds, the first method will do. For minutes or hours, the second method is better.

Quick Fix

To quickly adjust the time, use ntpdate. Install it with:

yum install ntpdate

For a one-time sync:

/usr/sbin/ntpdate 1.north-america.pool.ntp.org

Use Crontab to set automatic periodic synchronization. For daily sync:

crontab -e
00 1 * * * /usr/sbin/ntpdate 1.north-america.pool.ntp.org

To sync on boot instead of at regular intervals:

crontab -e
@reboot /usr/sbin/ntpdate 1.north-america.pool.ntp.org

Gradual Correction

To change the time on the server gradually, use ntp in CentOS. Install it:

yum install ntp

Enable the service on startup:

chkconfig ntpd on

In the file /etc/ntp.conf, specify accurate time sources, for example:

server 0.north-america.pool.ntp.org
server 1.north-america.pool.ntp.org iburst
server 2.north-america.pool.ntp.org
server 3.north-america.pool.ntp.org

The iburst parameter works the same as in Ubuntu/Debian — it improves accuracy by sending a burst of packets.

Restart the service after making changes:

/etc/init.d/ntp restart

Then restart the daemon:

/etc/init.d/ntpd start

Additional Options

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Time synchronization is usually done with the server closest to your server geographically. But in the configuration, you can specify the desired region directly in the subdomain. For example:

asia.pool.ntp.org

europe.pool.ntp.org

Even if the NTP server is offline, it can still pass on system time. Just add this line:

server 127.127.1.0

You can also restrict access for external clients. By default, these parameters are set:

restrict -4 default kod notrap nomodify nopeer noquery
restrict -6 default kod notrap nomodify nopeer noquery

The options notrap, nomodify, nopeer, and noquery prevent changes to the server's configuration. KOD (kiss of death) adds another layer of protection: if a client sends requests too frequently, it receives a warning packet and then is blocked.

If you want to allow unrestricted access for the local host:

restrict 127.127.1.0

To allow devices in a local network to sync with the server:

restrict 192.168.1.0 mask 255.255.255.0 nomodify notrap

After any changes, restart the service:

service restart ntp

To check the service’s operation, use the command:

ntpq -p

It will display a table showing the time source address, server stratum, last synchronization time, and other useful data.