Originally Posted by Kevin@Radstock

Hi

have a look at these two ways and adapt as needed.
1/ =SUM(OFFSET($AL$14,COUNTA(AL14:AL100)-1,0,-12,1))
2/ =SUM(OFFSET($AL$14,MATCH(DATE(2014,6,1),A14:A100,0 ),0,-12,1)), change the date as required to sum the previous 12 months from that date.
Note: The dates in A14:A37 are the 1st of each month & formatted as mmm yyyy.