On June 30, at precisely 23:59:60 UTC, the world will get a leap second.

Google Cloud Platform Blog gives you some details about the potential consequences. It might be useful or not depending on your use-cases, but it's short and instructive anyhow.

When do leap seconds happen?
By convention, leap seconds happen at the end of either June or December. However, unlike leap years, leap seconds do not happen at regular intervals, because the Earth's rotation speed varies irregularly in response to climatic and geological events. For example, the 2011 earthquake in Japan shortened the day by 1.8 microseconds by speeding up the Earth's rotation.

Read the full content in the Google Cloud Platform Blog post: Got a second? A leap second that is. Be ready for June 30th!