Choose FORMULA IS and enter the formula
=AND(A1<"",(A1-TODAY())<15)

It will higlight cell A1 if it contains a date which falls within the next
15 days (including dates in the past)

If you want to highlight dates only in the next 15 days use
=AND(A1<"",(A1-TODAY())<15,(A1-TODAY())0)

"Tammy S." wrote:

I would like to format a date cell when the date will expire in 15 days. If
there is no date, I don't want any formatting. How can I set the formatting
to meet both of these conditions?