I have a very large table of non-unique items shipped; the table contains
€œitem id€� and €œship date€� columns.

I need to find the number of unique €œitem id-s€� shipped in a given
timeframe, ex. 01/01/08 - 03/31/08

Ive tried using the commonly quoted solution:
=SUMPRODUCT((A1:A10<"")/COUNTIF(A1:A10,A1:A10&""))

Somehow, I cannot get the additional check for timeframe to work in the
above formula, something like:
=SUMPRODUCT((A1:A10<"")/COUNTIF(A1:A10,A1:A10&"")*(B1:B10 = C1)*( B1:B10
<= C2))

I would appreciate suggestions for array formula solutions.

Thank you.