Why do we have leap seconds?
As you know, usually a minute lasts for 60 seconds.
But every now and then, one minute can actually last for 61 seconds.
This is called a leap second, and we've used them since 1972, to help make sure the time on our watches matches how the Earth is rotating.
The latest leap second came at the very end of 2016. The year was in fact a second longer than you might have expected, with one extra second added to the minute from 11:59pm to 12:00am on New Year's Eve.
Leap seconds are added either at the end of June or at the end of December.
You might remember that in June 2015, Newsround reported that an extra second was being added to the time. The last leap second before that was in 2012.
They occur at irregular intervals like this as the Earth's rotation isn't completely predictable! And we only find out when they're going to happen six months in advance.
The 2016 leap second is our 28th leap second since we started having them.
For ideas about what you could do with your extra second, check out Newsround's top tips!