Sum column in datagridview???

I have datagridview with column "date" and column "numeric".

date(datetime)............ numeric(string)
1/2/2012 ..................... 4
2/2/2012 ..................... 5
2/3/2012 ..................... 6


I use this code to sum entire column "numeric"(sum=15).
Code:

but I want to sum numeric column by month( february=9,march=6).

Please help me.Sorry for bad english.