I have a dataset which has data for 5 cities of United States.Out of the 5 cities, 3 cities have data from 2008-2012 and 2 have from 2005 to 2012 at a monthly level. I do not want to consider data only from 2008-2012 but want to consider from 2005-2012. How can I back forecast the data for the 3 cities from 2005 -2007 at a monthly level.