Hi,
SInce Your source is a bex query, i would suggest you to include this requirement in bex side itself,
May be through offset ,....0 t0 -5....
This would help in keeping your BO part thin..
Also year end changes would be taken care in bex side with this approach,,
If you want to implement in BO side itself, I would suggest Relative Function,,,
BR
Prabhith..