Where Clause

I am trying to modify the code in an existing ASP page that displays records when it runs. The code in the where clause I wish to change is as follows:

where_clause = where_clause & "[DateWOCancelled] Is Null And IsNull([ACT_IS]) And ([PROP_IS] < Now())"

This works fine, but I need to have the clause also evaluate another field (Rev_IS) to see if it contains a date, and if it does check to see if it is < Now(). If the Rev_IS is not null, I don't care any longer about the date contained in Prop_IS. I only want to evaluate if Rev_IS < Now().