counting unique customers in a list by a specific date range

By the way, this equation works to count all unique values in the range, regardless of the date:

=SUM(IF(FREQUENCY(MATCH($B$2:$B$13,$B$2:$B$13,0),MATCH($B$2:$B$13,$B$2:$B$13,0))>0,1,0))

Any insights or ideas?