Average qty delivered

I have a problem as below. I have n items to deliver and delivery may or may not take place in some months. but I want average qty delivered/month in last 12 month.I have to get this figure any day .Say today I need this figure . If I check the sheet after 45 days from today, I need previous12 month average at that point of time. So I think it will be a combination of TODAY???

Item Qty delivered Date
ABC12 300 01-01-2012
ssdfs 500 01-04-2013
ABC12 100 12-12-2012
adsfsf 123 26-4-13
ssdfs 300 22-04-2012
ssdfs 500 26-04-2012
ABC12 300 26-5-2012

So in the column if I have formula for Item"ABC12" My answer will be 100+300/12
So in the column if I have formula for Item"ssdfs" My answer will be 500+300+500/12(since I want 12 month avg and not exact date to date..)

Pl see attached file.