See http://www.boulder.nist.gov/timefreq/faq/faq.htm for more information.
You can call (303) 499-7111 to hear NIST's correct time announcement.
In UTC at midnight
On December Thirty-One,
You'll find you get an extra
Second full of fun.
If you're not in Greenwich England
When the moment comes about,
You'll need to make adjustments
To the interval in doubt
At seven in the evening
(For those on EST),
Or six p.m. in Central,
Or five for MST.
Or four p.m. Pacific
Or wherever you may be,
Please thank your friendly NISTite
In Time and Frequency!
For our globe is spinning slower
(As we all are, I'm afraid),
While the Earth Rotation Service
Notes the seconds it's delayed.
Since we cannot speed the planet
We must compensate the clock,
And leap an extra second
To match our slowing rock.
(David Lovering, an electronics engineer in NIST's Information Technology Laboratory, has a penchant for poetry.)