Try this....

Select C2. In the conditional formatting dialog, add this formula:
=AND(LEN(A2)0,LEN(C2)0,SUMPRODUCT(--(C$1:C1=C2),--(A$1:A1(A2-180))))
Select the formatting options to fill the cell with red when this formula
returns TRUE, then click OK. Use Format Painter to copy this conditional
formatting to all of column C.

Hope this helps,

Hutch

"hj" wrote:

column A is my finished date, column C is my serial #. If I enter the same
serial # within 180 days of my finshed date what formula can I use to turn
cell red?