i don't really understand your formula here as I am not familiar with the numertodate() and it is not in my help file and your "then" output is the the same as your "else" output.
That being said, you can create a formula field with nothing in it called NULL. You can then call that formula field in your other formula.
I would rework this as it does not make sense to me but strictly following your request it would be:
if isnull({WOSUB.SUBCPD}) then
{@NULL} else if {WOSUB.SUBCPD} > 0 then numbertodate ({WOSUB.SUBCPD}) else numbertodate ({WOSUB.SUBCPD})
Edited by DBlank - 31 Jan 2012 at 3:48am