On 30 June, at 23:59:60 (00:59:60 BST), an extra second was added to Coordinated Universal Time (UTC). This is what is known as a leap second.
The slight change meant several high-profile sites which rely heavily on synchronised operations were disrupted.
I posted earlier about GPS and how GPS time (determined by the atomic clocks in the GPS satellites) is now extremely important to the world. I wondered how GPS time accounts for leap seconds. The answer is: it doesn’t. However, GPS does provide UTC by transmitting the necessary data about leap seconds in its signal so that UTC can be calculated from GPS time by a GPS receiver. I read about this here.