convert individual dates value into month as a sum.

I have some data in date wise.my requirement is------to convert all dates value into month.
example:
1-jan-1980-------------------------4(value),
23-jan-1980------------------------5(value)
28-jan-1980------------------------2(value);

final result/output:
jan-1980------------------11(value).
I am sharing my data sheet here.

thanks for your quick reply.But it will be huge help if u show in my excel file how to use SUMIF function.