Space.com: Climate change and polar ice melting could be impacting the length of Earth's day

Currently, we keep official time using around 450 ultra-precise atomic clocks to keep Coordinated Universal Time (UTC), which was first defined in 1969. A more traditional historical method of time-keeping uses the rotation of Earth. But because Earth's rotation fluctuates, since 1972, the alignment between these two measurements has been maintained by adding 27 "leap seconds" to the official time standard.