Accurately measuring time can be a puzzle. The most common time period is the day, measured by a complete rotation of the earth. But modern technology, not to mention train schedules, rely on precise timekeeping. The earth's rotation is a bit inexact, so we switched to measuring seconds using the resonance of cesium atoms in 1955. Since then, the varying spin rate of the earth makes it necessary to occasionally add a "leap second" to a year to keep things straight.
But in the 21st century, melting glaciers at the poles are adding volume to the earth's oceans, which changes the distribution of the globe's mass, and is beginning to slow down the earth's rotation. Scientists are looking to the possibility that we may have to employ negative leap seconds to the year, meaning we would be subtracting a second instead of adding one. That may sound simple, but computers and the systems that depend on computers aren't built for that change. If that sounds confusing, the conundrum is explained in a thorough and understandable way at Smithsonian.