AWS Compute Blog

It’s About Time: Microsecond-Accurate Clocks on Amazon EC2 Instances

This post is written by Josh Levinson, AWS Principal Product Manager and Julien Ridoux, AWS Principal Software Engineer

Today, we announced that we improved the Amazon Time Sync Service to microsecond-level clock accuracy on supported Amazon EC2 instances. This new capability adds a local reference clock to your EC2 instance and is designed to deliver clock accuracy in the low double-digit microsecond range within your instance’s guest OS software. This post shows you how to connect to the improved clocks on your EC2 instances. This post also demonstrates how you can measure your clock accuracy and easily generate and compare timestamps from your EC2 instances with ClockBound, an open source daemon and library.

In general, it’s hard to achieve high-fidelity clock synchronization due to hardware limitations and network variability. While customers have depended on the Amazon Time Sync Service to provide one millisecond clock accuracy, workloads that need microsecond-range accuracy, such as financial trading and broadcasting, required customers to maintain their own time infrastructure, which is a significant operational burden, and expensive. Other clock-sensitive applications that run on the cloud, including distributed databases and storage, have to incorporate message exchange delays with wait periods, data locks, or transaction journaling to maintain consistency at scale.

With global and reliable microsecond-range clock accuracy, you can now migrate and modernize your most time-sensitive applications in the cloud and retire your burdensome on-premises time infrastructure. You can also simplify your applications and increase their throughput by leveraging the high-accuracy timestamps to determine the ordering of events and transactions on workloads across instances, Availability Zones, and Regions. Additionally, you can audit the improved Amazon Time Sync Service to measure and monitor the expected microsecond-range accuracy.

New improvements to Amazon Time Sync Service

The new local clock source can be accessed over the existing Amazon Time Sync Service’s Network Time Protocol (NTP) IPv4 and IPv6 endpoints, or by configuring a new Precision Time Protocol (PTP) reference clock device, to get the best accuracy possible. It’s important to note that both NTP and the new PTP Hardware Clock (PHC) device share the same highly accurate source of time. The new PHC device is part of the AWS Nitro System, so it is directly accessible on supported bare metal and virtualized Amazon EC2 instances without using any customer resources.

A quick note about Leap Seconds

Leap seconds, introduced in 1972, are occasional one-second adjustments to UTC time to factor in irregularities in Earth’s rotation to UTC time in order to accommodate differences between International Atomic Time (TAI) and solar time (Ut1). To manage leap seconds on behalf of customers, we designed leap second smearing within the Amazon Time Sync Service (details on smearing time in “Look Before You Leap”).

Leap seconds are going away, and we are in full support of the decision made at the 27th General Conference on Weights and Measures to abandon leap seconds by or before 2035.

To support this transition, we still plan on smearing time when accessing the Amazon Time Sync Service over the local NTP connection or our Public NTP pools ( The new PHC device, however, will not provide a smeared time option. In the event of a leap seconds, PHC would add the leap seconds following UTC standards. Leap smeared and leap second time sources are the same in most cases. But, since they differ during a leap second event, we do not recommend mixing smeared and non-smeared time sources in your time client configuration during a leap second event.

Connect using NTP (automatic for most customers)

You can connect to the new, microsecond-accurate clocks over NTP the same way you use the Amazon Time Sync Service today at the IPv4 address or the fd00:ec2::123 IPv6 address. This is already the default configuration on all Amazon AMIs and many partner AMIs, including RHEL, Ubuntu, and SUSE. You can verify this connection in your NTP daemon. The below example, using the chrony daemon, verifies that chrony is using the IPv4 address of the Amazon Time Sync Service to synchronize the time:

[ec2-user@ ~]$ chronyc sources
MS Name/IP address         Stratum Poll Reach LastRx Last sample
^-              3  10   377    69  +5630us[+5632us] +/-   90ms
^-           2   8   377   224   -691us[ -689us] +/-   33ms
^*               1   4   377     2  -4487ns[-5914ns] +/-   85us
^-         2   9   377   327  -1710us[-1720us] +/-   64ms
^-                 2   9   377   161  +3057us[+3060us] +/-   84ms

The IPv4 address of the Amazon Time Sync Service is designated with a *, showing it is the source of synchronization on this instance. See the EC2 User Guide for more details on configuring the Amazon Time Sync Service if it is not already configured by default.

Connect using the PTP Hardware Clock

First, you need to install the latest Elastic Network Adapter (ENA) driver. This driver will allow you to connect directly to the PHC. Connect to your instance and install the Linux kernel driver for Elastic Network Adapter (ENA) version 2.10.0 or later. For the installation instructions, see Linux kernel driver for Elastic Network Adapter (ENA) family on GitHub. To enable PTP support in the driver follow the instructions in the section “PTP Hardware Clock (PHC)“.

Once the driver is installed, you need to configure your NTP daemon to connect to the PHC. Below is an example on how to change the configuration in chrony by adding the PHC to your chrony configuration file. Then restart chrony for the change to take place:

[ec2-user ~]$ sudo sh -c 'echo "refclock PHC /dev/ptp0 poll 0 delay 0.000010 prefer" >> /etc/chrony.conf'
[ec2-user ~]$ sudo systemctl restart chronyd

This example uses a +/-5 microsecond range in receiving the reference signal from the PHC. These 10 microseconds are needed to account for operating system latency.

After changing your configuration, you can validate your daemon is correctly syncing to the PHC. Below is an example of output from the chronyc command. An asterisk will appear next to the PHC0 source indicating that you are now syncing to the PHC:

[ec2-user@ ~]$ chronyc sources
MS Name/IP address         Stratum Poll Reach LastRx Last sample
#* PHC0                           0   0   377     1   +18ns[  +20ns] +/- 5032ns

The PHC0 device of the Amazon Time Sync Service is designated with a *, showing it is the source of synchronization on this instance

Your chrony tracking information will also show that you are syncing to the PHC:

[ec2-user@ ~]$ chronyc tracking
Reference ID    : 50484330 (PHC0)
Stratum         : 1
Ref time (UTC)  : Mon Nov 13 18:43:09 2023
System time     : 0.000000004 seconds fast of NTP time
Last offset     : -0.000000010 seconds
RMS offset      : 0.000000012 seconds
Frequency       : 7.094 ppm fast
Residual freq   : -0.000 ppm
Skew            : 0.004 ppm
Root delay      : 0.000010000 seconds
Root dispersion : 0.000001912 seconds
Update interval : 1.0 seconds
Leap status     : Normal

See the EC2 User Guide for more details on configuring the PHC.

Measuring your clock accuracy

Clock accuracy is a measure of clock error, typically defined as the offset to UTC. This clock error is the difference between the observed time on the computer and the reference time (also known as true time). If your instance is configured to use the Amazon Time Sync Service where the microsecond-accurate enhancement is available, you will typically see a clock error bound of under 100us using the NTP connection. When configured and synchronized correctly with the new PHC connection, you will typically see a clock error bound of under 40us.

We previously published a blog on measuring and monitoring clock accuracy over NTP, which still applies to the improved NTP connection.

If you are connected to the PHC, your time daemon, such as chronyd, will underestimate the clock error bound. This is because inherently, a PTP hardware clock device in Linux does not pass any “error bound” information to chrony, the way the NTP would. As a result, your clock synchronization daemon assumes the clock itself is accurate to UTC and thus has an “error bound” of 0. To get around this issue, the Nitro System calculates the error bound of the PTP Hardware Clock itself, and exposes it to your EC2 instance over the ENA driver sysfs filesystem. You can read this directly as a value in nanoseconds with the command cat /sys/devices/pci0000:00/0000:00:05.0/phc_error_bound. To get your clock error bound at some instant, you would need to add the clock error bound from chrony or ClockBound at the time that chronyd polls the PTP Hardware Clock and add it to this phc_error_bound value.

Below is how you would calculate the clock error incorporating the PHC clock error to get your true clock error bound:


For the values in the example:

PHC Error Bound = cat /sys/devices/pci0000:00/0000:00:05.0/phc_error_bound

The System Time, Root Delay, and Root Dispersion are values taken from the chrony tracking information.


However accurate, a clock is never perfect. Instead of providing an estimate of the clock error, ClockBound provides a reliable confidence interval by automatically calculating the clock accuracy, using the calculations in which the reference time (true time) does exist. The open source ClockBound daemon provides a convenient way to retrieve this confidence interval, and work is continuing to make it easier to integrate into high performance workloads.


The Amazon Time Sync Service’s new microsecond-accurate clocks can be leveraged to migrate and modernize your most clock-sensitive applications in the cloud. In this post, we showed you how to can connect to the improved clocks on supported Amazon EC2 instances, how to measure your clock accuracy, and how to easily generate and compare timestamps from your Amazon EC2 instances with ClockBound. Launch a supported instance and get started today to build using this new capability.

To learn more about the Amazon Time Sync Service, see the EC2 UserGuide for Linux and Windows.

If you have questions about this post, start a new thread on the AWS Compute re:Post or contact AWS Support.

Hear about the Amazon Time Sync Service at re:Invent

We will speak in more detail about the Amazon Time Sync Service during re:invent 2023. Look for Session ID CMP220 in the AWS re:Invent session catalog to register.