Today a positive leap second will be introduced, as per IERS announcement:
To authorities responsible for the measurement and distribution of time: UTC TIME STEP on the 1st of July 2015. A positive leap second will be introduced at the end of June 2015.
The sequence of dates of the UTC second markers will be:
2015 June 30, 23h 59m 59s
2015 June 30, 23h 59m 60s
2015 July 1, 0h 0m 0s
National Geographic has a good video on this leap second phenomenon:
The International Earth Rotation and Reference Systems Service adds a leap second every few years to keep the clocks we use to measure official time and the speed of Earth’s rotation in sync. But why do we need to do this? And what kind of problems could result?
Watch your digital clocks, they should seem to be stuck for a second at midnight (UTC) 🙂