Earth will complete a full rotation in slightly less time than usual on Tuesday, July 22, making it one of the shortest days ever recorded.

The difference will be just 1.34 milliseconds less than the standard 24 hours — not something you'll notice — but it's part of a puzzling trend in Earth's rotational behavior that has been unfolding in recent years. If it continues, a second may need to be subtracted from atomic clocks around 2029 — a so-called negative leap second, which has never been done before.