Copied from AWS user guide:
Amazon Linux instances are set to the UTC (Coordinated Universal Time) time zone by default
Network Time Protocol (NTP) is configured by default on Amazon Linux instances; however, an instance needs access to the Internet for the standard NTP configuration to work. In addition, your instance’s security group rules must allow outbound UDP traffic on port 123 (NTP), and your network ACL rules must allow both inbound and outbound UDP traffic on port 123. […] If your instance does not have access to the Internet, you need to configure NTP to query a different server in your private network to keep accurate time.
solved Amazon summer time reliability