As it turns out, New Year’s will be happening tonight a bit later than people expected, and you can thank NASA. The space agency has added a “leap second” to the Master Clock just before midnight on Dec. 31 on Coordinated Universal Times, which actually corresponds to 6:59:59 p.m. Eastern time.

Why is NASA making this decision? The agency carefully corresponds its clocks to the Earth’s rotation. Since the Earth gradually slows down over time, that means periodic adjustments. In fact, during the time of the dinosaurs, a day was only 23 hours, NASA says in a statement.

Scientists used the Solar Dynamics Observatory (SDO), which is constantly focused on the sun, to make the adjustment. The adjustment is necessary to keep the SDO in sync with Coordinated Universal Time.

NASA does this periodically. They last added a second to June 30, 2015, and also made the adjustment on 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.”