What causes leap seconds? Earthquakes, tidal drag, the weather On June 30th at precisely 23:59:59, the world’s atomic clocks will pause for a single second. Or, to be more precise, they’ll change to the uncharted time of 23:59:60 — before ticking over to the more worldly hour of 00:00:00 on the morning of July 1st, 2015. This addition of a leap second, announced by the Paris Observatory this week , is being added to keep terrestrial clocks in step with the vagaries of astronomical time — in this case, the slowing of the Earth’s rotation. when the last leap second was added back in 2012, more than a few sites had trouble keeping pace. As reported by Phys.org , Foursquare, Reddit, LinkedIn, and StumbledUpon all crashed when the leap second ticked unexpectedly into place. via 2015 is getting an extra second and that's a bit of a problem for the internet | The Verge . Also read :: June 30th is a second too long to keep clocks accurate The leap secon...