Having an extra day in a leap year is not unusual but 2016 not only gave us an extra day but also an extra second meaning a slightly longer wait to bring in 2017.

The reason? Leap years are in place to keep us aligned with the different seasons whereas leap seconds occur are more for astronomical purposes. That extra second allows our Earth clocks to synchronize with the Sun’s position in accordance with Earth’s rotation. The last time this happened was June 30. 2015.

An extra 26 seconds have been added since leap seconds were first introduced in 1972 but it is not always easy to predict when we need one. The Earth moves quite erratically meaning it’s impossible to tell way in advance when a leap second needs to be added. Therefore, they need to observed closely and added on a as-needed basis and so far only leap seconds have been added rather than taken away.

However, some are calling to have leap seconds abolished due to the complexity of hardware and software technologies. Every program needs to readjust itself which could potentially cause serious problems if they were to get it wrong such as navigation failures and computer crashes.

But this doesn’t stop the fact that the Earth slows down by 2 milliseconds a day which means every 500 days requires an extra second.

“By adding a leap second, we’re essentially stopping our atomic clocks to let the Earth catch up,” explained USNO astronomer, Geoff Chester.