New Year’s has been postponed. But chances are, unless you work for NASA, you probably didn’t notice it when you rung in the New Year at midnight. That’s because the space agency merely added a second to the Master Clock before midnight on Dec. 31.

This “leap second” was added to the Coordinated Universal Time in order to correspond its clocks to Earth’s rotation. The Earth is actually gradually slowing down as time marches on, so the clock must be periodically tweaked to keep it accurate. Way back when dinosaurs roamed the Earth, a day was just 23 hours.

Scientists made their decision based on data from the Solar Dynamics Observatory (SDO), a satellite that focuses on the sun. Scientists make this adjustment to the Master Clock every now and then. They did it back on June 30, 2015, and also June 30, 2012.

“SDO moves about 1.9 miles every second,” said Dean Pesnell, the project scientist for SDO at NASA’s Goddard Space Flight Center in Greenbelt, Maryland, in a statement. “So does every other object in orbit near SDO. We all have to use the same time to make sure our collision avoidance programs are accurate. So we all add a leap second to the end of 2016, delaying 2017 by one second.”

The NASA statement adds: “The leap second is also key to making sure that SDO is in sync with the Coordinated Universal Time, or UTC, used to label each of its images. SDO has a clock that counts the number of seconds since the beginning of the mission. To convert that count to UTC requires knowing just how many leap seconds have been added to Earth-bound clocks since the mission started. When the spacecraft wants to provide a time in UTC, it calls a software module that takes into consideration both the mission’s second count and the number of leap seconds — and then returns a time in UTC.”