Sum of stock returns with date constraint

[fransden]

Dear Excelforum

I am about to creat momentum porfolios and need help to sum daily returns for each stock in the months t-12 to t-1. For example the cell january 01 2009 should sum the returns for the first trading day january 2008 to the last trading day novemer 2008, thereafter I want to roll this function so, do the same for the next month, first trading day february 2009 should sum the returns for first trading day february 2008 to last trading day december 2008.

I have tried with an if formula but I am not sure how to set the constraints.

Please see attached for example data.

I am very grateful for your help!

/Fransden