The 2nd of September 2014.
120 days from May 5 2010 is Thursday, September 2, 2010.
In any given year, the date 120 days after May 15 is September 12.
Adding 120 days to May 11, 2010 gives September 8, 2010.
Adding 120 days to May 20, 2010 gives September 17, 2010.
Adding 120 days to May 29, 2010 gives September 26, 2010.
January 17 + 120 days = May 17 (except in a leap year).
Thursday, May 12th, 2011 falls 120 day after January 12th, 2011.
Adding 120 days to January 28, 2011 gives May 28, 2011.
Subtracting 120 days from September 13, 2010 gives May 16, 2010.
Adding 120 days to January 27, 2011 gives May 27, 2011.
Adding 120 days to January 9, 2012 gives May 8, 2012.
120 days after Mon, May 31, 2010 = Tue, Sep 28, 2010.