I think its from the day you took delivery unless you are purchasing a service loaner.
2013 X3 35i, Dealer Delivery July 2012
2011 550i M Sport, European Delivery May 2011
Currently in Transit to Dealer: 2014 550i M Sport, European Delivery May 2014
Gone, but not forgotten:
2006 530i, European Delivery June 2005
2008 335i Coupe 6MT, PCD November 2007