Setting the Time for Your Linux Instance

A consistent and accurate time reference is crucial for many server tasks and processes. Most system logs include a time stamp that you can use to determine when problems occur and in what order the events take place. If you use the AWS CLI or an AWS SDK to make requests from your instance, these tools sign requests on your behalf. If your instance's date and time are not set correctly, the date in the signature may not match the date of the request, and AWS rejects the request.

Amazon provides the Amazon Time Sync Service, which is accessible from all EC2 instances, and is also used by other AWS services. This service uses a fleet of satellite-connected and atomic reference clocks in each Region to deliver accurate current time readings of the Coordinated Universal Time (UTC) global standard through Network Time Protocol (NTP). The Amazon Time Sync Service automatically smooths any leap seconds that are added to UTC.

The Amazon Time Sync Service is available through NTP at the 169.254.169.123 IP address for any instance running in a VPC. Your instance does not require access to the internet, and you do not have to configure your security group rules or your network ACL rules to allow access. The latest versions of Amazon Linux 2 and Amazon Linux AMIs synchronize with the Amazon Time Sync Service by default.

Use the following procedures to configure the Amazon Time Sync Service on your instance using the chrony client. Alternatively, you can use external NTP sources. For more information about NTP and public time sources, see http://www.ntp.org/. An instance needs access to the internet for the external NTP time sources to work.

Configuring the Amazon Time Sync Service on Amazon Linux AMI

Note

On Amazon Linux 2, the default chrony configuration is already set up to use the Amazon Time Sync Service IP address.

With the Amazon Linux AMI, you must edit the chrony configuration file to add a server entry for the Amazon Time Sync Service.

To configure your instance to use the Amazon Time Sync Service

Connect to your instance and uninstall the NTP service.
```
[ec2-user ~]$ sudo yum erase 'ntp*'
```
Install the chrony package.
```
[ec2-user ~]$ sudo yum install chrony
```
Open the /etc/chrony.conf file using a text editor (such as vim or nano). Verify that the file includes the following line:
```
server 169.254.169.123 prefer iburst minpoll 4 maxpoll 4
```
If the line is present, then the Amazon Time Sync Service is already configured and you can go to the next step. If not, add the line after any other server or pool statements that are already present in the file, and save your changes.
Start the chrony daemon (chronyd).
```
[ec2-user ~]$ sudo service chronyd start
```
```
Starting chronyd:                                          [  OK  ]
```
Note

On RHEL and CentOS (up to version 6), the service name is chrony instead of chronyd.
Use the chkconfig command to configure chronyd to start at each system boot.
```
[ec2-user ~]$ sudo chkconfig chronyd on
```

Verify that chrony is using the 169.254.169.123 IP address to synchronize the time.


[ec2-user ~]$ chronyc sources -v


210 Number of sources = 7

  .-- Source mode  '^' = server, '=' = peer, '#' = local clock.
 / .- Source state '*' = current synced, '+' = combined , '-' = not combined,
| /   '?' = unreachable, 'x' = time may be in error, '~' = time too variable.
||                                                 .- xxxx [ yyyy ] +/- zzzz
||      Reachability register (octal) -.           |  xxxx = adjusted offset,
||      Log2(Polling interval) --.      |          |  yyyy = measured offset,
||                                \     |          |  zzzz = estimated error.
||                                 |    |           \
MS Name/IP address         Stratum Poll Reach LastRx Last sample               
===============================================================================
^* 169.254.169.123               3   6    17    43    -30us[ -226us] +/-  287us
^- ec2-12-34-231-12.eu-west>     2   6    17    43   -388us[ -388us] +/-   11ms
^- tshirt.heanet.ie              1   6    17    44   +178us[  +25us] +/- 1959us
^? tbag.heanet.ie                0   6     0     -     +0ns[   +0ns] +/-    0ns
^? bray.walcz.net                0   6     0     -     +0ns[   +0ns] +/-    0ns
^? 2a05:d018:c43:e312:ce77:>     0   6     0     -     +0ns[   +0ns] +/-    0ns
^? 2a05:d018:dab:2701:b70:b>     0   6     0     -     +0ns[   +0ns] +/-    0ns

In the output that's returned, ^* indicates the preferred time source.

Verify the time synchronization metrics that are reported by chrony.


[ec2-user ~]$ chronyc tracking


Reference ID    : A9FEA97B (169.254.169.123)
Stratum         : 4
Ref time (UTC)  : Wed Nov 22 13:18:34 2017
System time     : 0.000000626 seconds slow of NTP time
Last offset     : +0.002852759 seconds
RMS offset      : 0.002852759 seconds
Frequency       : 1.187 ppm fast
Residual freq   : +0.020 ppm
Skew            : 24.388 ppm
Root delay      : 0.000504752 seconds
Root dispersion : 0.001112565 seconds
Update interval : 64.4 seconds
Leap status     : Normal

Configuring the Amazon Time Sync Service on Ubuntu

You must edit the chrony configuration file to add a server entry for the Amazon Time Sync Service.

To configure your instance to use the Amazon Time Sync Service

Connect to your instance and use apt to install the chrony package.
```
ubuntu:~$ sudo apt install chrony
```
Note

If necessary, update your instance first by running sudo apt update.
Open the /etc/chrony/chrony.conf file using a text editor (such as vim or nano). Add the following line before any other server or pool statements that are already present in the file, and save your changes:
```
server 169.254.169.123 prefer iburst minpoll 4 maxpoll 4
```

Restart the chrony service.


ubuntu:~$ sudo /etc/init.d/chrony restart


[ ok ] Restarting chrony (via systemctl): chrony.service.

Verify that chrony is using the 169.254.169.123 IP address to synchronize the time.


ubuntu:~$ chronyc sources -v


210 Number of sources = 7

  .-- Source mode  '^' = server, '=' = peer, '#' = local clock.
 / .- Source state '*' = current synced, '+' = combined , '-' = not combined,
| /   '?' = unreachable, 'x' = time may be in error, '~' = time too variable.
||                                                 .- xxxx [ yyyy ] +/- zzzz
||      Reachability register (octal) -.           |  xxxx = adjusted offset,
||      Log2(Polling interval) --.      |          |  yyyy = measured offset,
||                                \     |          |  zzzz = estimated error.
||                                 |    |           \
MS Name/IP address         Stratum Poll Reach LastRx Last sample
===============================================================================
^* 169.254.169.123               3   6    17    12    +15us[  +57us] +/-  320us
^- tbag.heanet.ie                1   6    17    13  -3488us[-3446us] +/- 1779us
^- ec2-12-34-231-12.eu-west-     2   6    17    13   +893us[ +935us] +/- 7710us
^? 2a05:d018:c43:e312:ce77:6     0   6     0   10y     +0ns[   +0ns] +/-    0ns
^? 2a05:d018:d34:9000:d8c6:5     0   6     0   10y     +0ns[   +0ns] +/-    0ns
^? tshirt.heanet.ie              0   6     0   10y     +0ns[   +0ns] +/-    0ns
^? bray.walcz.net                0   6     0   10y     +0ns[   +0ns] +/-    0ns

In the output that's returned, ^* indicates the preferred time source.

Verify the time synchronization metrics that are reported by chrony.


ubuntu:~$ chronyc tracking


Reference ID    : 169.254.169.123 (169.254.169.123)
Stratum         : 4
Ref time (UTC)  : Wed Nov 29 07:41:57 2017
System time     : 0.000000011 seconds slow of NTP time
Last offset     : +0.000041659 seconds
RMS offset      : 0.000041659 seconds
Frequency       : 10.141 ppm slow
Residual freq   : +7.557 ppm
Skew            : 2.329 ppm
Root delay      : 0.000544 seconds
Root dispersion : 0.000631 seconds
Update interval : 2.0 seconds
Leap status     : Normal

Configuring the Amazon Time Sync Service on SUSE Linux

Install chrony from https://software.opensuse.org/package/chrony.

Open the /etc/chrony.conf file using a text editor (such as vim or nano). Verify that the file contains the following line:


server 169.254.169.123 prefer iburst minpoll 4 maxpoll 4

If this line is not present, add it. Comment out any other server or pool lines. Open yast and enable the chrony service.

Changing the Time Zone on Amazon Linux

Amazon Linux instances are set to the UTC (Coordinated Universal Time) time zone by default, but you may wish to change the time on an instance to the local time or to another time zone in your network.

Important

These procedures are intended for use with Amazon Linux. For more information about other distributions, see their specific documentation.

To change the time zone on an instance

Identify the time zone to use on the instance. The /usr/share/zoneinfo directory contains a hierarchy of time zone data files. Browse the directory structure at that location to find a file for your time zone.
```
[ec2-user ~]$ ls /usr/share/zoneinfo
Africa      Chile    GB         Indian       Mideast   posixrules  US
America     CST6CDT  GB-Eire    Iran         MST       PRC         UTC
Antarctica  Cuba     GMT        iso3166.tab  MST7MDT   PST8PDT     WET
Arctic      EET      GMT0       Israel       Navajo    right       W-SU
...
```
Some of the entries at this location are directories (such as America), and these directories contain time zone files for specific cities. Find your city (or a city in your time zone) to use for the instance. In this example, you can use the time zone file for Los Angeles, /usr/share/zoneinfo/America/Los_Angeles.
Update the /etc/sysconfig/clock file with the new time zone.
1. Open the /etc/sysconfig/clock file with your favorite text editor (such as vim or nano). You need to use sudo with your editor command because /etc/sysconfig/clock is owned by root.
2. Locate the ZONE entry, and change it to the time zone file (omitting the /usr/share/zoneinfo section of the path). For example, to change to the Los Angeles time zone, change the ZONE entry to the following:
```
ZONE="America/Los_Angeles"
```
  Note
  
  Do not change the UTC=true entry to another value. This entry is for the hardware clock, and does not need to be adjusted when you're setting a different time zone on your instance.
3. Save the file and exit the text editor.
Create a symbolic link between /etc/localtime and your time zone file so that the instance finds the time zone file when it references local time information.
```
[ec2-user ~]$ sudo ln -sf /usr/share/zoneinfo/America/Los_Angeles /etc/localtime
```
Reboot the system to pick up the new time zone information in all services and applications.
```
[ec2-user ~]$ sudo reboot
```

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