try this

=AND(A3<TODAY(),(ERROR.TYPE(B3)=7))

On Oct 21, 8:51*pm, Lost in Microbiology
wrote:
I have an expected due date in column A and an actual date in column B (if
there is no data a #N/A appears). I have a conditional format set to remind
me of upcoming, but am having trouble with one to tell me about past due.

A * * * * * * * *B
6/26/08 * * * 6/25/08
9/24/08 * * * #N/A
3/20/08 * * * 3/25/08
11/18/08 * * #N/A

My first condition is: =AND(A3<TODAY()+30,A3=TODAY())

This one works fine, it highlights the cells of upcoming appointments.

My Second condition is: =AND(A3<TODAY(),A3="#N/A")

The second condition is not doing anything. I am missing something easy,
thanks for your help.