Today is going to be one of the longest days of your life, according to international timekeepers who have added a "leap second" to the end of the day. To account for the gradual slowing down of the Earth's rotation, today will have 86,401 seconds instead of the usual 86,400, NASA says in a press release, explaining that the slowdown is caused by a "kind of braking force caused by the gravitational tug of war between Earth, the moon, and the sun." Instead of going from 23:59:59 to 00:00:00 at midnight tonight, clocks on Coordinated Universal Time—aka "atomic time"—will go to 23:59:60 first.
This is the fourth leap second this century and the first since 2012, when the extra second brought down a large chunk of the Internet. "Whenever a leap second occurs, some computer systems encounter problems due to glitches in the code written to handle them," a scientist at the UK's National Physical Laboratory tells the BBC. "The consequences are particularly severe in the Asia-Pacific region, where leap seconds occur during normal working hours." It's hard to predict when leap seconds will occur because so many factors, including weather patterns and changes in the Earth's core, affect our planet's rotation, according to NASA, which notes that abandoning them for a different way of accounting for the slowdown will be discussed at a conference this year.