Today – December 21, 2014 – is the day of the winter solstice in the northern hemisphere, the summer solstice in the southern hemisphere. Astronomically, this solstice is the precise time when the Sun is over the Tropic of Capricorn and the poles are at their maximum tilt in relation to the Sun.
That will happen today at 11:03 PM UTC, which is the same as Greenwich Mean Time at this time of year. In New York, it will be five hours earlier at 5:03 PM. But for time zones east of the Greenwich Meridian, the solstice will occur on December 22nd.
By the way, even though this is the shortest day in the northern hemisphere, the mornings will still get darker for a few weeks. Earth's orbit isn't a perfect circle and we move at fluctuating speeds in orbit. So the Sun may lag behind the clock or be ahead of it, but it works out on average!