Formula to count unique strings broken down by month

Originally Posted by AlKey

It was a rounding issue.

Yes, because you can't reliably use SUMPRODUCT/COUNTIF to count uniques with a condition.

Use this array formula**:

=SUM(IF(FREQUENCY(IF(TEXT(B$2:B$241,"mmmm")=E3,MATCH(A$2:A$241,A$2:A$241,0)),ROW(A$2:A$241)-ROW(A$2)+1),1))

** array formulas need to be entered using the key
combination of CTRL,SHIFT,ENTER (not just ENTER).
Hold down both the CTRL key and the SHIFT key
then hit ENTER.