Year: 2015

Each EC2 instance has its own clock and is fully under your control; AWS does not manage instance clocks. An instance clock can be affected by many factors. Depending on these factors, it may implement or skip the leap second. It may also be isolated and not synchronize to an external time system. If you need your EC2 instance clocks to be predictable, you can use NTP to synchronize your clocks to time servers of your choice. For more information about how to synchronize clocks, see the following documentation:

• Instances using Amazon Linux AMIs: Setting the Time for Your Linux Instance.
• Instances using Amazon-provided Microsoft Windows AMIs: Setting the Time for a Windows Instance.
• Instances using other AMIs: Please contact your AMI provider (the information in the preceding bullet points may also be helpful).

Adding the leap second is currently the standard practice. If you use public time servers, like time servers from ntp.org (the default for Amazon Linux AMIs) or time.windows.com (the default for Amazon Windows AMIs), your instance will see the leap second unless these synchronization services announce a different practice.

Source: Look Before You Leap – The Coming Leap Second and AWS | AWS Official Blog

A reminder to check how (y)our EC2 instances are going to deal with this before it happens at the end of June.