https://aws.amazon.com/blogs/aws/keeping-time-with-amazon-time-sync-service/

synchronised to NTP server (169.254.169.123) at stratum 3
   time correct to within 4 ms
   polling server every 16 s

AWS make an NTP server available on the IPv4 link-local address range (169.254.0.0/16) in the same way that their metadata service is. It can therefore always be reached by compute instances that have a propertly configured network connection, regardless of the EC2 security groups and VPC routing tables.

https://docs.aws.amazon.com/AWSEC2/latest/UserGuide/configuring-instance-metadata-service.html