Everybody knows about leap years: once every four years, we throw an extra day into the calendar so that we can laugh about the poor people born on February 29th who can't buy beer because they're only six years old. If you paid attention in school, you might even know that every year divisible by 100 is not a leap year, unless it’s also divisible by 400.
Okay, Mr. Look Who Knows So Much, how about a leap second? Thing is, while we keep track of time with atomic clocks that degrade in a stable pattern, the Earth’s rotation isn’t quite that even. So every once in while, we need to stick in an extra second to get everything to even out. This year is just such a year, so don’t freak out if you notice the clock lagging a bit towards midnight. It's perfectly normal, and doesn't in any way signal the imminent appearance of hideous creatures from below the Earth's surface. That's next year.
Of course, if you’re running OS X and you’ve got your Mac pulling time from Apple’s time servers (that’s the default setting in the Date & Time preference pane), you’re taken care of. And if you’re on Windows, well, er, we'll let Microsoft explain:
The Windows Time service does not indicate the value of the Leap Indicator when the Windows Time service receives a packet that includes a leap second. (The Leap Indicator indicates whether an impending leap second is to be inserted or deleted in the last minute of the current day.) Therefore, after the leap second occurs, the NTP client that is running Windows Time service is one second faster than the actual time. This difference is resolved at the next time synchronization.
Uh. We think that means you’re fine. But maybe you want to keep a fire extinguisher on hand…just in case.