in column O i have a date in format dd/mm/yy. In column P I want to put
a formula to say:

if O is blank then do nothing

if date in O is not in the past then cell value= number of days from
today

if date in O IS in the past then cell value = "Expired"

I already have conditional formatting set up to colour the cells in
based on the date, so now I just need to fill them in.

I know its not hugely difficult, I just cant ever get the formulas to
work properly hehe

here still another possibility:
Formula in P1:
= If(O1="", "",O1-TODAY())

Use the user-defined numberformat:
0;[<0]"Expired";

--
Mit freundlichen Grüssen

Melanie Breden
- Microsoft MVP für Excel -

http://excel.codebooks.de (Das Excel-VBA Codebook)