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P H wrote:

http://www.msnbc.msn.com/id/35662192/ns/technology_and_science-space/?GT1=43001

And then, this paragraph caught my attention:
"One Earth day is about 24 hours long. Over the course of a year, the length of a day normally changes gradually by one millisecond. It increases in the winter, when the Earth rotates more slowly, and decreases in the summer, Gross has said in the past."

The seasonal variation of the earth rotation has been noticed as early as 1936 by Adelsberger, employing the quartz clock that he developed with Scheibe at the Physikalisch-Technische Anstalt in Berlin. The phenomenon has subsequently been confirmed by others. According to Seidelmann, Explanatory Supplement to the Astronomical  Almanac, 1992, p. 85  (which see for further details and literature references) it has been found to agree.with lunar tidal variations.

Said seasonal variation define the difference between UT2 and UT1. For the exact term, see Seidelmann, loc. cit, or also

http://www.cv.nrao.edu/~rfisher/Ephemerides/times.html#UT2

Herbert Prinz