SUMPRODUCT with COUNTIF to find unique entries

Using your posted workbook,

This ARRAY FORMULA, completed by holding down CTRL and SHIFT when you press ENTER (instead of just pressing ENTER), returns the count of unique Customer ID's for May

D3: =COUNT(1/FREQUENCY(IF(B2:B17=F3,MATCH(F3&A2:A17,B2:B17&A2:A17,0)),ROW(A2:A17)-1))

In your example, that formula returns: 4

Is that something you can work with?

Thanks Fotis and Ron. Is there anyway to solve this apart from using VBA and CSE formula? I used to use CSE Formula, but boss is not able to understand and thus I have to redo my work.