June 30, 2015 will officially be a
bit longer than usual because an extra second, or 'leap' second, will be added
to the day, according to NASA.
Strictly speaking, a day lasts 86,400 seconds. That is the
case, according to the time standard that people use in their daily lives -
Coordinated Universal Time, or UTC. UTC is "atomic time" - the
duration of one second is based on extremely predictable electromagnetic
transitions in atoms of cesium. These transitions are so reliable that the
cesium clock is accurate to one second in 1,400,000 years.
