MDX Except PeriodtoDate

So I have two sets of data that I get from PeriodtoDate functions.

Lets say first PeriodtoDate gives me set from 1/1/2012 till 1/31/2012 --- Monthly

and second periodtodate gives me set from 1/1/2012 till 1/15/2012  -- Daily

Now what I want is

1) the result set from 1/15/2012 ti 1/31/2012.

2)  And then take an average out of them.

Can the Except function help me in this to achieve step 1. If yes, how can I form it, specially knowing that one set is based on monthly computation and the other one is daily.

Thanks

-Sarah