From Leap second on June 30 is no Y2K bug, says expert:

Most days last for 86,400 seconds, but June 30, 2015, will last for 86,401.

Because the Earth doesn’t rotate at a constant speed, scientists have taken to adding the occasional leap second to keep international timekeeping in line with the world’s super-accurate atomic clocks.

The last leap second, added on June 30, 2012, caused trouble for some organizations. Flight reservation systems failed at Australia’s Qantas Airlines, causing flight delays. Reddit, the popular online message board, was knocked offline after its Linux-based servers failed to cope with the leap second. Other websites were similarly affected.