Directly from our Department of Homeland Security (apparently this is a big deal for REALLY accurate people):




1. Leap Second Introduction
The Coordinated Universal Time (UTC) time standard, based on atomic clocks, is widely used for international timekeeping and as the reference for time in most countries. UTC is the basis of legal time for most of the world.
UTC must be adjusted at irregular intervals to maintain its correlation to mean solar time due to irregularities in the Earth’s rotation. These adjustments, called leap seconds, are pre-determined. The next leap second will occur on 30 June 2015 at 23:59:59 UTC.
Specifically, a positive leap second will be inserted between second 23:59:59 of 30 June and second 00:00:00 of 1 July. This extra second is displayed on UTC clocks as 23:59:60.

So, everyone, prepare yourselves...