If today feels like it’s going by slower than usual, it’s because it is. A leap second will be added to today’s time in order to keep the Earth’s rotation in sync with the atomic clock. As NASA explains it, scientists once analyzed radio waves sent from far off quasars to determine that our planet’s rotation is gradually slowing down due to gravitational forces from the Sun and moon—adding two milliseconds to each day. In order to recalibrate our clocks, a leap second is added every couple of years, usually at the end of June or December. This year, on 30 June, be sure to catch your clock hitting 23:59:60 before entering July.