The 13th of March is 180 days before it. The 8th of March is 180 days after it, except when that March is in a leap year, in which case the 7th of March is 180 days after it.
Adding 180 days to September 9 on a non-leap year gives March 7 of the following year. Adding 180 days to September on a leap year gives March 6 of the following year.
March 7, 2012
180 days from September 14, 2011 is March 23, 2011.
Adding 180 days to September 20, 2010 gives March 19, 2011.
180 days from November 9, 2012 would be May 8, 2013.
Adding 180 days to September 20, 2008 gives March 19, 2009.
Adding 180 days to September 17, 2010 gives March 16, 2011.
Adding 180 days to September 28, 2011 gives March 26, 2012.
Adding 180 days to September 13 leading into a non-leap year gives March 12 of the following year. Adding 180 days to September 13 leading into a leap year gives March 11 of the following year.
September 1, 2010 + 180 days = February 28, 2011.
Adding 180 days to March 19 gives September 15.
180 days after Wed, Mar 9, 2011 = Mon, Sep 5, 2011