A second longer.

Adapted from Yahoo! News.

On Dec. 31 this year, your day will be just a second longer.

Like the more well-known time adjustment, the leap year, a "leap second" is tacked on to clocks every so often to keep them correct.

Earth's trip around the sun - our year with all its seasons - is about 365.2422 days long, which we round to 365 to keep things simpler. But every four years, we add 0.2422 x 4 days (that's about one day) at the end of the month of February (extending it from 28 to 29 days) to fix the calendar.

Likewise, a "leap second" is added on to our clocks every so often to keep them in synch with the somewhat unpredictable nature of our planet's rotation, the roughly 24-hour whirl that brings the sun into the sky each morning.

Historically, time was based on the mean rotation of the Earth relative to celestial bodies and the second was defined from this frame of reference. But the invention of atomic clocks brought about a definition of a second that is independent of the Earth's rotation and based on a regular signal emitted by electrons changing energy state within an atom.

In 1970, an international agreement established two timescales: one based on the rotation of the Earth and one based on atomic time.