Calculate Value based on Days in Month

Hi.

I am looking to calculate budget values based on days in a month.

Fields!Budget.Value * 12/52 This gives me the weekly budget.

I need per day budget.

easy enough...

Fields!Budget.Value/31 =budget per day

The issue is not every month has a neat 31 days.

Can someone tell me how to do this in SSRS.?

Thanks for the your help in advance!

Ziggies