A Slow Moving Stock report

We are trying to build a report that identifies slow moving stock and calculates how long the stock would be expected to last on the shelf for, based on previous sales.

I can't see how to achieve this and wonder if anyone here has any ideas?

We need to be able to 1) Calculate how old the oldest unit is in stock (by showing the oldest Item Receipt date when comparing current on hand qty against the cumulative totals of recent item receipt quantities)

We then need to use this figure along with the quantity sold in the last, say, 3 months, to work out how long it would take to sell the remaining quantity that are on hand.