This summer will be a little bit longer than usual. A tiny little bit: one second, to be precise. The world’s official time keepers are adding a single second to the clocks at the end of June. This “leap second” is needed to keep various time scales in synch. It’s a bit of a pain and won’t really affect people much, but if it weren’t done things would get messy eventually.
This article, from the blog at Discover Magazine, goes on to explain why there is a need to add a second into the scheme and explains the consequences of allowing the two measures of time to diverge.