The Wolfram Language function AbsoluteTime counts the number of seconds from Jan 1, 1900, while UNIX time is the number of seconds from Jan 1,1970.
You can convert pretty easily with the following:
DateList[1409855505 + AbsoluteTime[{1970}]]
which returns your expected:
{2014, 9, 4, 18, 31, 45.}
