AWS EC2 instances and the coming leap second

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:

Adding the leap second is currently the standard practice. If you use public time servers, like time servers from (the default for Amazon Linux AMIs) or (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.