The program calculates trends in temperature anomalies — not absolute temperatures — but changes relative to the average temperature for the same month during the period of 1951 - 1980. (giss.nasa.gov)
One can also calculate the trends over successive periods of, say, ten years, with start - points separated by one year. (realclimate.org)
The minimum number of years required to calculate a trend value is 18. (ipcc.ch)