Reporting on products 'active' between certain dates

Hi all, 

On my report I have products with a start and end date, and I want to track throughout the year how many products were active in each month. Is it a case of establishing a seperate column/table of dates that are unconnected to the product start dates and then using this as a comparison to report back on how many are 'active' in any given month before they end? 

I've been playing with this for a while, and think I must be missing something obvious, therefore any help would be appreciated with regards to the logic I need to apply. 

Thanks!