SQL & PL/SQL

Can I put a subquery in the where clause, on the left side on the operator?
This is a multi-row query.

Like this,

   select a.col1, a.col2, b.col1, b.col2,
             my_function(a.date1, b.date2) AS GROSSDAYS
   from table1 a
   where ( select ( a.date1 - b.date2 ) as range
               from     table1 a
               join      table2 b
               on       a.col3 = b.col3
               where rownum =1
           )
               in ( 1,2,3)
and  a.col1 = b.col2

I need to use a subquery because the column I need does not exist in the table, and I cannot make any changed to the table structure.

Is what I'm doing possible?

The subquery is the same as the function I have in the Select clause.