How are you defining "then drill down to weeks, etc."?

Week 1= Jan 1-6, week 2 = jan 7-13?

What about leap years?

I could be wrong but I recommend you rethink your approach here.

There are some fundamental issues you have going on that just won't work.

You want to drill down. Drilling down is drilling into grouped data but you are not really grouping data you are just showing values (which could be based on groups).

Also the per week function is a Sunday to Saturday. If you are also grouping by Month and year at a higher level it will break up your weeks. So as you try to compare weeks from year to year or month to month you are giving out odd information as the Sun-Sat alters year to year so your comparisons are off.

Is this making sense?