Source: http://www.theatlantic.com/technology/ar...

In early November, the orbital movement towards perihelion coincides with the Earth's obliquity to put the Sun more than 16 and a half minutes ahead of where it “should” be if the Earth were in a not-tilted, perfectly circular orbit. This offset is called the “Equation of Time” (equation, in this sense, being an old fashioned term meaning ”adjustment”). As a result, the sunrise and the sunset are earlier than they should otherwise be. This +16 minute offset rapidly dwindles as the Earth continues its orbit: the Sun is 14 minutes ahead by mid-November, and only 10 minutes ahead by early December. This adjustment is large enough to cause the earliest sunset to occur during the first week of December.

By December 21, the sunset is about three minutes later than it was in early December. But, spurred on by the double effect of the perihelion change and the tilt change, the sunrise has been getting later as well, gaining nearly fifteen minutes. This means that despite all of the time shifting, the solstice is still the day with the shortest amount of daylight.