Convert weekly p&l data to accurate monthly data

I want to take a weekly p&l forecast (complete) and create a monthly summary for it. I want it to accurately reflect the number of days in each month in 2015. For instance, I don't just want to sum the first four week forecast in 2015 and call it January - I want to pull the first 31 days of the forecast (so the first four weeks + 3/7 of the fifth week). Needs to be dynamic so every time the weekly p&l is updated, the monthly summary is as well. I know this is a simple set of helper rows and index formulas... I just don't have time to do it. Need completion quickly. Thank you!