This June the world is getting a 'leap second' to compensate for the slowing of the Earth's rotation around the Sun.
After 11:59:59pm on June 30 2015, the clock will strike 11:59:60pm - which could cause problems for some parts of the internet.
The last time the world added a leap second, in 2012, it took down major websites including Reddit, Foursquare, Yelp and LinkedIn. The problem? Most computer systems not being programmed to deal with an unexpected extra second.
To avoid this issue Google add fractions of a second to programmes over the year before a leap second, in a move known as a "leap smear".
Scientists at the Paris Observatory and the International Earth Rotation Service decided to add an extra second to help "Earth time" catch up to "atomic time" and this will be the 26th leap second since 1972.