Actually, it's traditional in meteorological statistics to start the
year on December 1, so that the winter season (traditionally DJF) isn't
split across two years.

They might have waited with the press release another couple of weeks
to keep this confusion out of the foreground, but there's nothing
underhanded about it. The data for 1998, the previous record holder
also ran from Dec 1 of the prior year to Nov 30.

See http://data.giss.nasa.gov/gistemp/1998/

"A meteorological year runs from the beginning of winter to the end of
autumn, and so the 1998 meteorological year started on Dec. 1, 1997,
and ended on Nov. 30, 1998".

mt