Precision clock and time synchronization on your EC2 instance

A consistent and accurate time reference on your Amazon EC2 instance is crucial for many server tasks and processes. Time stamps in system logs play an essential role in identifying when issues occurred and the chronological order of events. When 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 settings are inaccurate, it can result in a discrepancy between the date in the signature and the date of the request, leading to AWS rejecting your requests.

To address this important aspect, Amazon offers the Amazon Time Sync Service, which is accessible from all EC2 instances and used by various AWS services. The service uses a fleet of satellite-connected and atomic reference clocks in each AWS Region to deliver accurate and current time readings of the Coordinated Universal Time (UTC) global standard.

For the best performance, we recommend using the local Amazon Time Sync Service on your EC2 instances. For a backup to the local Amazon Time Sync Service on your instances, or to connect resources outside of Amazon EC2 to the Amazon Time Sync Service, you can use the public Amazon Time Sync Service located at time.aws.com. The public Amazon Time Sync Service, like the local Amazon Time Sync Service, automatically smears any leap seconds that are added to UTC. The public Amazon Time Sync Service is supported globally by our fleet of satellite-connected and atomic reference clocks in each AWS Region.

Leap seconds

Leap seconds, introduced in 1972, are occasional one-second adjustments to UTC time to factor in irregularities in the earth’s rotation 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. For more information, see Look Before You Leap – The Coming Leap Second and AWS.

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 during a leap second event when accessing the Amazon Time Sync Service over the local NTP connection or our public NTP pools (time.aws.com). The PTP hardware clock, however, does not provide a smeared time option. In the event of a leap second, the PTP hardware clock will add the leap second following UTC standards. Leap-smeared and leap second time sources are the same in most cases. But because they differ during a leap second event, we do not recommend using both smeared and non-smeared time sources in your time client configuration during a leap second event.