The length of day and the seasons has to do with the tilt of the Earth's axis and its annual trip around the Sun. This journey takes 364.25 days, so every four years we need to add a Leap Day to the calendar. You would think that this is all confusing enough. Changing the number of days in a year, switching time zones without moving twice a year, but now we have to adjust our clocks again!
On June 30, 2012, a “leap second” will be added to the world’s clocks at 23 hours, 59 minutes, 59 seconds Coordinated Universal Time (UTC). This corresponds to 7:59:59 p.m. Eastern Daylight Time, when the extra second will be inserted at the U.S. Naval Observatory’s Master Clock Facility in Washington, DC.
Historically, time was based on the rotation of the Earth relative to celestial bodies, and the second was defined in this reference frame. However, the invention of atomic clocks defined a much more precise “atomic” timescale and a second that is independent of Earth’s rotation. In 1970, an international agreement established two timescales: one based on the rotation of the Earth, known as UT1, and one based on atomic time, Coordinated Universal Time, or UTC. The International Earth Rotation and Reference Systems Service (IERS) is the organization which monitors the difference in the two time scales and calls for leap seconds to be inserted in or removed from UTC when necessary to keep them within 0.9 second of each other.
The common misconception is that the occasional insertion of leap seconds every few years indicates the rate at which the Earth's rotation is slowing. Now you know the real story.
Don't worry. Your work day isn't going to get any longer, and our calendar with all its warts and blemishes will continue to serve us well into the future. This tiny adjustment isn't going to have much, if any impact on your life. It might be interesting to watch your smartphone or GPSUTC clock do a slight hiccup just before 8pm on June 30th. For all my fellow obsessive compulsive friends out there, this is an excellent time to reset your clocks and watches to be strictly accurate.
I just hope I'm having a good day on June 30, because at 8pm I'm going to have to relive a whole second of it!