The slowing rotation of the earth is problematic. June 30th Gets a Leap Second Because Earth's Rotation is Slowing Down.
Most of us live our lives in the steadfast world of coordinated universal time (UTC), where Earth days are treated as precisely 86,400 seconds long. But in the real world, days haven’t been that long since about 1820. That’s because a gravitational tug-of-war between the Earth and the moon is causing our planet’s rotation to slow down, making the days a wee bit longer as the years roll on. Today, the average day is approximately 86,400.002 seconds long.
Because computer programmers like to rely on atomic clocks without really bothering to account for all of the things that atomic clocks do (like inserting the odd leap second), this thing can crash computers. The moral of the story is, “Do your homework.” Don’t assume that you know how things really work. Because you probably missed a few details.
There have been 24 leap seconds since 1972, but that doesn’t mean that computer programmers know about it – or take the correct action.