C3 or I6 will never be blank so this doesn't work for me.

It's harmless to include the additional checks on C3/I6 being blank**.
The earlier suggestion (below) should still work fine:
=IF(OR($C$3="",I6="",I6<$C$3),"",<your formula)
since the 3rd check: I6<$C$3 will take care of your reqt
(I'm not sure that you did try it out?)

It could of course, be reduced to just this:
=IF(I6<$C$3,"",<your formula)

**the precaution is because blank cells are evaluated as zeros in formulas,
and dates are just numbers, so if I6 is blank and C3 contains a date, then
the check I6<$C$3 would misleadingly return TRUE.
--
Max
Singapore
http://savefile.com/projects/236895
xdemechanik
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