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    Elastic Compute Service:Precision clock and time synchronization on ECS instance

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    Elastic Compute Service:Precision clock and time synchronization on ECS instance

    Last Updated:Oct 31, 2025

    Time discrepancies can result in scheduled task errors, inconsistent log timestamps, and clock skew in distributed systems. To address these aspects, you can calibrate the time for your Elastic Compute Server (ECS) instance by configuring a clock synchronization service.

    How it works

    image

    • Network Time Protocol (NTP): A protocol for synchronizing clocks between a client and a server to within milliseconds by calculating network latency.

    • Alibaba Cloud NTP Server: Provides Virtual Private Cloud (VPC) endpoints that serve as highly accurate time sources for your ECS instances, allowing them to synchronize their system clocks with minimal network latency.

    • chrony/ntpd/W32time: Time synchronization services that use NTP to synchronize an instance's system clock with a time reference provided by an NTP server. On Linux/Unix systems, chrony offers better performance than ntpd. Windows systems use the built-in W32time service.

    Verify clock accuracy

    Quick guide

    Linux (chrony)

    ECS instances created from Alibaba Cloud public images run chrony by default. Some older public images run ntpd by default. To check which process is running, use ps -C chronyd,ntpd -o comm=. If the output is ntpd, see Linux (ntpd) tab.

    Run the command below to check the clock offset. In the output, look for the System time field. Its value shows the current offset between your instance's system clock and the NTP time reference. In this example, 0.000027851 seconds fast of NTP time means the system clock is 0.000027851 seconds ahead of the NTP server time. If this offset exceed your application's tolerance, adjust your clock configuration.

    To check the clock accuracy on multiple instances, use Cloud Assistant Batch Check.
    sudo chronyc tracking
    Reference ID    : 64643D58 (100.100.61.88)
Stratum         : 2
Ref time (UTC)  : Tue Oct 28 05:53:58 2025
System time     : 0.000027851 seconds fast of NTP time
Last offset     : +0.000044533 seconds
RMS offset      : 0.000036478 seconds
Frequency       : 41.053 ppm slow
Residual freq   : +0.000 ppm
Skew            : 0.007 ppm
Root delay      : 0.000408892 seconds
Root dispersion : 0.011003596 seconds
Update interval : 1028.9 seconds
Leap status     : Normal

    Linux (ntpd)

    Run the command below to check the clock offset. In the output, look for the offset field. Its value shows the difference between your instance's system clock and the NTP time reference, in milliseconds. Compare this offset with your application's tolerance. If it exceeds your application's tolerance, adjust your clock configuration.

    To check the clock accuracy on multiple instances, use Cloud Assistant Batch Check.
    sudo ntpq -p
    remote           refid      st t when poll reach   delay   offset  jitter
==============================================================================
 LOCAL(0)        .LOCL.          10 l  107   64    2    0.000    0.000   0.000
+47.96.XXX.XXX   100.100.61.91    2 u   37   64    3   31.241  1009.95   1.313
 11.116.XXX.XX   .INIT.          16 u    -   16    0    0.000    0.000   0.000
-100.100.3.1     10.137.55.181    2 u   52   64    7   25.344  1010.26   0.566
-100.100.3.2     10.137.55.181    2 u   18   64    7   26.873  1008.80   2.143
-100.100.3.3     10.137.55.181    2 u   66   64   12   29.656  1010.16   0.902
-203.107.6.88    10.137.55.181    2 u   65   64    7   33.689  1005.74   0.697
 11.196.XXX.XXX  .INIT.          16 u    -   16    0    0.000    0.000   0.000
 11.196.XXX.XX   .INIT.          16 u    -   16    0    0.000    0.000   0.000
 10.143.0.44     .INIT.          16 u    -   16    0    0.000    0.000   0.000
 10.143.0.45     .INIT.          16 u    -   16    0    0.000    0.000   0.000
 10.143.0.46     .INIT.          16 u    -   16    0    0.000    0.000   0.000
+100.100.5.1     203.107.X.XX     2 u   59   64    7   53.214  1009.42   0.519
*100.100.5.2     203.107.X.XX     2 u   58   64    7   49.062  1009.26   0.984
-100.100.5.3     203.107.X.XX     2 u   57   64    7   56.670  1011.81   3.331

    Windows

    Run w3tm /query /status command in the Command Prompt window. The Root Dispersion field in the output shows the current clock offset. In this example, 7.7612440s means the system clock is 7.7612440 seconds ahead of the NTP server time. If this offset exceed your application's tolerance, adjust your clock configuration.

    To check the clock accuracy on multiple instances, use Cloud Assistant Batch Check.
    Leap Indicator: 0(no warning)
Stratum: 3 (secondary reference - syncd via (S)NTP)
Precision: -23 (119.209ns per tick)
Root Delay: 0.0259591s
Root Dispersion: 7.7612440s
Reference ID: 0x64640302 (source IP:  100.100.3.2)
Last Successful Sync Time: 2025/10/28 15:05:02
Source: ntp11.cloud.aliyuncs.com,0x9
Poll Interval: 6 (64s)

    Configure clock synchronization

    Linux (chrony)

    ECS instances created from Alibaba Cloud public images run chrony by default. Some older public images run ntpd by default. To check which process is running, use ps -C chronyd,ntpd -o comm=. If the output is ntpd, see Linux (ntpd) tab.

    To configure clock synchronization on multiple instances, use Cloud Assistant Batch Execution.

    1. Check and configure NTP server address

    1. Edit the chrony configuration file.

      Alibaba Cloud Linux/CentOS/RHEL

      sudo vim /etc/chrony.conf

      Ubuntu/Debian

      sudo vim /etc/chrony/chrony.conf

    2. Configure Alibaba Cloud NTP servers.

      Remove or comment out lines starting with server, then copy the following content into the configuration file.

      Using multiple NTP servers provides redundancy and improves time accuracy by allowing the client to select the best server.
      # Configuration information, including NTP server addresses, minimum polling interval, and maximum polling interval.
server ntp.cloud.aliyuncs.com minpoll 4 maxpoll 10 iburst
server ntp10.cloud.aliyuncs.com minpoll 4 maxpoll 10 iburst
server ntp11.cloud.aliyuncs.com minpoll 4 maxpoll 10 iburst
server ntp12.cloud.aliyuncs.com minpoll 4 maxpoll 10 iburst
server ntp7.cloud.aliyuncs.com minpoll 4 maxpoll 10 iburst
server ntp8.cloud.aliyuncs.com minpoll 4 maxpoll 10 iburst
server ntp9.cloud.aliyuncs.com minpoll 4 maxpoll 10 iburst

    3. Save the configuration file and restart chrony.

      sudo systemctl restart chronyd.service

    2. Verify clock synchronization

    After restarting, verify the time synchronization metrics.

    sudo chronyc tracking

    The System time in the output shows the offset between your system's clock and the NTP server's time. For example, 0.000036150 seconds fast of NTP time indicates a small discrepancy.

    If the time offset is large (for example, several minutes or hours) and your workload can tolerate an abrupt time change, consider manually synchronizing the clock.
    Reference ID    : 64643D58 (100.100.61.88)
Stratum         : 2
Ref time (UTC)  : Sat Sep 20 07:55:33 2025
System time     : 0.000036150 seconds fast of NTP time
Last offset     : +0.000031322 seconds
RMS offset      : 0.000044841 seconds
Frequency       : 43.592 ppm slow
Residual freq   : +0.000 ppm
Skew            : 0.005 ppm
Root delay      : 0.000228760 seconds
Root dispersion : 0.010518964 seconds
Update interval : 1029.1 seconds
Leap status     : Normal

    Linux (ntpd)

    Certain legacy Alibaba Cloud public images, such as CentOS 6, are pre-configured with the ntpd service. Because the NTP service is no longer actively maintained, we recommend upgrading to Chrony for better performance and security.

    To configure clock synchronization on multiple instances, use Cloud Assistant Batch Execution.

    1. Check and configure NTP server addresses

    1. Edit the NTP configuration file.

      sudo vim /etc/ntp.conf

    2. Configure Alibaba Cloud NTP servers.

      Remove or comment out lines starting with server, then copy the following content into the configuration file.

      Using multiple NTP servers provides redundancy and improves clock accuracy by allowing the client to select the best server.
      # Configuration information, including NTP server addresses, minimum polling interval, and maximum polling interval.
server ntp10.cloud.aliyuncs.com iburst minpoll 4 maxpoll 10
server ntp11.cloud.aliyuncs.com iburst minpoll 4 maxpoll 10
server ntp12.cloud.aliyuncs.com iburst minpoll 4 maxpoll 10
server ntp7.cloud.aliyuncs.com iburst minpoll 4 maxpoll 10
server ntp8.cloud.aliyuncs.com iburst minpoll 4 maxpoll 10
server ntp9.cloud.aliyuncs.com iburst minpoll 4 maxpoll 10

    3. Save the configuration file and restart ntpd.

      Alibaba Cloud Linux/CentOS/RHEL

      sudo service ntpd restart

      Ubuntu/Debian

      service ntp restart

    2. Verify clock synchronization

    After restarting, verify the time synchronization metrics. Key parameters in the output:

    • offset: the offset between your system's clock and the NTP server's time, in milliseconds. A value closer to 0 indicates higher accuracy.

    • jitter: The variation in time delay. A smaller value indicates more precise clock synchronization.

    If the time offset is large (for example, several minutes or hours) and your workload can tolerate an abrupt time change, consider manually synchronizing the clock.
    sudo ntpq -p
         remote           refid      st t when poll reach   delay   offset  jitter
==============================================================================
 LOCAL(0)        .LOCL.          10 l   17   64    1    0.000    0.000   0.000
+100.100.3.1     10.137.55.181    2 u    8   16    1   30.426  -307.76   0.153
*100.100.3.2     10.137.55.181    2 u    5   16    1   26.845  -309.25   0.221
+100.100.3.3     10.137.55.181    2 u    4   16    1   28.819  -309.37   0.187
-100.100.5.1     203.107.X.XX     2 u    3   16    1   50.478  -310.70   0.164
-100.100.5.2     203.107.X.XX     2 u    2   16    1   47.235  -311.89   0.246
 100.100.5.3     203.107.X.XX     2 u    1   16    1   53.661  -310.50   0.119

    Windows

    Graphical user interface (GUI)

    Change the default NTP server to an Alibaba Cloud NTP server. The following steps use Windows Server 2022 as an example.

    1. In the notification area of the taskbar, click the date and time, and then click Date and time settings.

      image

    2. On the Settings page, click Related settings > Add clocks for different time zones.

    3. In the Date and Time dialog box, click Internet Time > Change settings...

    4. In the Internet Time Settings dialog box, select Synchronize with an Internet time server, enter ntp.cloud.aliyuncs.com, click Update now, and then click OK.

    5. Verify the synchronization. A success message confirms that the time has been updated. If the synchronization fails, check if a firewall or security software is blocking UDP port 123.

      image

    Command-line interface (CLI)

    To configure clock synchronization on multiple instances, use Cloud Assistant Batch Execution.

    1. Configure NTP servers

    Change the default NTP server to an Alibaba Cloud NTP server. The following steps use Windows Server 2022 as an example.

    1. Open a Command Prompt window.

    2. Set the NTP server. The following example uses ntp11.cloud.aliyuncs.com.

       w32tm /config /manualpeerlist:"ntp11.cloud.aliyuncs.com" /syncfromflags:manual /reliable:YES /update

    3. Restart the time service.

       net stop w32time
 net start w32time

    4. Synchronize clock with the NTP server.

      w32tm /resync

    2. Verify clock synchronization

    If the Source field in the output is Local CMOS Clock instead of the configured NTP server address, the synchronization has failed.

     w32tm /query /status
    Leap Indicator: 0(no warning)
Stratum: 3 (secondary reference - syncd via (S)NTP)
Precision: -23 (119.209ns per tick)
Root Delay: 0.0020371s
Root Dispersion: 0.0191713s
Reference ID: 0x64640302 (source IP:  100.100.3.2)
Last Successful Sync Time: 2025/10/28 14:37:42
Source: ntp11.cloud.aliyuncs.com,0x9
Poll Interval: 10 (1024s)

    FAQ

    How can I manually force a clock synchronization for Linux/Unix systems?

    To avoid the risks associated with sudden time jumps, we do not recommend manual synchronization. However, if the time offset is significant (several minutes or hours) and your workload can tolerate a sudden time jump, you can manually synchronize the clock.

    Linux (chrony)

    1. Stop chrony.

      sudo systemctl stop chronyd.service

    2. Manually synchronize the clock.

      sudo chronyd -q

    3. Start chrony.

      sudo systemctl start chronyd.service

    Linux (ntpd)

    Alibaba Cloud Linux/CentOS/RHEL

    1. Stop ntpd.

      sudo service ntpd stop

    2. Manually synchronize the clock.

      sudo service ntpdate start

    3. Start ntpd.

      sudo service ntpd start

    Ubuntu/Debian

    1. Stop ntpd.

      sudo service ntp stop

    2. Manually synchronize the clock.

      sudo ntpdate ntp.cloud.aliyuncs.com ntp7.cloud.aliyuncs.com ntp10.cloud.aliyuncs.com ntp11.cloud.aliyuncs.com ntp12.cloud.aliyuncs.com

    3. Start ntpd.

      sudo service ntp start

    What are the Alibaba Cloud NTP server domain names?

    The following table lists the Alibaba Cloud NTP server addresses for different networks (IPv4 only).

    • For Alibaba Cloud ECS Instances: Use the VPC domains for lower network latency.

    • For non-Alibaba Cloud instances: Use the Internet domains.

    VPC domains

    Internet domains

    ntp.cloud.aliyuncs.com

    ntp7.cloud.aliyuncs.com

    ntp8.cloud.aliyuncs.com

    ntp9.cloud.aliyuncs.com

    ntp10.cloud.aliyuncs.com

    ntp11.cloud.aliyuncs.com

    ntp12.cloud.aliyuncs.com

    ntp.aliyun.com

    ntp1.aliyun.com

    ntp2.aliyun.com

    ntp3.aliyun.com

    ntp4.aliyun.com

    ntp5.aliyun.com

    ntp6.aliyun.com

    ntp7.aliyun.com

    Is the Alibaba Cloud NTP service free of charge?

    Yes, the Alibaba Cloud NTP service is free. Public images are pre-configured for clock synchronization, and instances created from them automatically use chrony or ntpd to keep their clock accurate.

    How do I upgrade from ntpd to chrony?

    Some older Alibaba Cloud ECS images, such as CentOS 6, use ntpd. Since ntpd is no longer actively maintained, we recommend upgrading to chrony unless your workload has a specific dependency on ntpd.

    1. Uninstall the ntp service.

      Alibaba Cloud Linux/CentOS/RHEL
      sudo yum remove ntp
      Ubuntu/Debian
      sudo apt remove ntp

    2. Install Chrony.

      If the download fails, see How to fix yum failures when installing chrony on CentOS 6 (EOL)?

      Alibaba Cloud Linux/CentOS/RHEL
      sudo yum install chrony
      Ubuntu/Debian
      sudo apt update

sudo apt install chrony

    3. Configure Chrony.

      Edit the configuration file.

      Alibaba Cloud Linux/CentOS/RHEL

      sudo vim /etc/chrony.conf

      Ubuntu/Debian

      sudo vim /etc/chrony/chrony.conf

      Add NTP servers and other settings to the file. The following is a simple chrony.conf example:

      # NTP server settings
server ntp.cloud.aliyuncs.com minpoll 4 maxpoll 10 iburst
# Specify the location of the drift file
driftfile /var/lib/chrony/drift
# Allow chronyd to be queried from localhost
allow 127.0.0.1

      Parameter descriptions:

      Parameter

      Description

      server <address>

      The domain name or IP address of the NTP server. chrony connects to this server to synchronize clock.

      minpoll <n>

      The minimum polling interval, specified as a power of 2 in seconds. minpoll 4 means the minimum interval is 2⁴=16 seconds.

      maxpoll <n>

      The maximum polling interval, specified as a power of 2 in seconds. maxpoll 10 means the maximum interval is 2¹⁰=1024 seconds.

      iburst

      An option that speeds up the initial synchronization. When chrony starts, it sends a burst of requests to quickly achieve accurate time.

    4. Start the chrony service.

      1. Start the Chrony service and enable it to start on boot.

        # Start the chrony service
sudo service chronyd start

# Set chrony to start automatically when the system boots
sudo chkconfig chronyd on

      2. Check the chrony service status.

        # Check the service status
sudo service chronyd status

      3. Verify the clock synchronization with the chronyc tool.

        sudo chronyc tracking

    How to fix yum failures when installing chrony on CentOS 6 (EOL)?

    1. Back up and remove the old repo configuration files.

      sudo mkdir -p /etc/yum.repos.d/backup && sudo mv /etc/yum.repos.d/*.repo /etc/yum.repos.d/backup/

    2. Create a new repository configuration file that points to the official CentOS vault.

      sudo tee /etc/yum.repos.d/CentOS-Vault.repo <<'EOF'
[C6.10-base]
name=CentOS-6.10 - Base
baseurl=http://vault.centos.org/6.10/os/$basearch/
gpgcheck=1
gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-CentOS-6
enabled=1
[C6.10-updates]
name=CentOS-6.10 - Updates
baseurl=http://vault.centos.org/6.10/updates/$basearch/
gpgcheck=1
gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-CentOS-6
enabled=1
[C6.10-extras]
name=CentOS-6.10 - Extras
baseurl=http://vault.centos.org/6.10/extras/$basearch/
gpgcheck=1
gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-CentOS-6
enabled=1
EOF

    3. Clean and rebuild the yum cache.

      sudo yum clean all && sudo yum makecache

    4. Install chrony.

      sudo yum install chrony

    Reference

    To learn more about Linux time concepts and operations, or to understand changes to clock configurations in Linux Public Images, see Linux time and time zones.