I am a real estate agent trying to find trends, or create relational grapghs in apartment data in excel 2007.

A B C
1 unit date closing_price
2 #1001 2/11/2008 $1,932,507
3 #1002 9/12/2012 $1,775,000
4 #1002 7/8/2011
5 #1002 1/28/2008 $1,656,436
6 #1003 9/16/2010 $1,335,000
7 #1003 5/21/2008 $1,303,233


1) For every apartment (column A) I would like to determine how many years or how many months there is between sale dates in column B
2) then I would like to determine for all apartments what is the average length between sale dates in column B.