On June 30, the official Universal Coordinated Time will experience a leap second. Many technical news sites have written about this upcoming leap second’s impact on various technical systems.
System Administrators concerned about leap seconds should focus on it as a standard maintenance problem of updating Java’s time zone information and using operating system time tools like NTP.
Time Zone updates were covered in a previous post, Understanding Time Zone Updater 2.0. System Administrators looking to update their systems can simply use the tzupdater tool to update JDK installations. Alternately, each JDK release always contains the latest time zone information for users that upgrade.
Java relies on the operating system clock for time. Each operating system has its own method of handling date changes such as leap seconds. Consult your operating system information for details on handling the leap second. Linux users may consult the Wired interview with Linus Torvalds, "Linux’s creator wants us all to chill out about the leap second."
Details for Windows can be found in KB-909614 How the Windows Time service treats a leap second. Hosting providers like Amazon and Google will be doing a time skew over the course of a day.
Although the Java API defines a second as a number between 0 and 61 to accommodate leap seconds, some applications may have taken short-cuts to assume that there are 60 seconds in a minute or 86400 seconds in a day. Systems that perform synchronized time-based transactions with other systems may run into issues if one system updates and another does not.
The 2015 leap second takes place at exactly 23:59:60 UTC on June 30, 2015. I advise system administrators to monitor processes during this time and pay extra attention to systems for a short while.