correlation by subset

Hi all

Having searched the available documentation on R, I request for help
in sorting out the underlying problem.

I have a huge dataset containing 2 variables x and y, which is a daily
price series.

I would like to observe the quarterly correlations among these two
variables. Is there anyway where i can calculate cor.coeff by using a
grouping variable in R.

thanks in advance for the help

rgds

snvk