No, really. Today will have 86,401 seconds in it, as opposed to the usual 86,400 seconds that every day for the last 18 years has had.
Because the earth interacts with lots of other gravity sources in the universe—most notably the moon—its rotation sometimes speeds up and sometimes slows down. Over the last 18 years or so, the planet has lost an entire second because of these perturbations, requiring us to update our most accurate clocks to compensate. Of course, when those clocks get updated, there's a trickle-down effect, because so much of what we do in the 21st Century requires really, really accurate timekeeping.
So, this evening in Chicago, the 6pm hour will have 3,601 seconds in it as the master clocks all over the planet add their leap second at 23:59:60 UTC.
Enjoy your extra second.