USage of Date Functions in Where Clause

I am using a query which looks like

Select col1, col2,
fn_date_covert(dat_col1) - fn_date_covert(dat_col2) differnce
From table1, table2
where join (table1, table2)
and fn_date_covert(dat_col1) - fn_date_covert(dat_col2) >=1;

though this query works, but its takiling lot of time...
I feel it is taking time bcause of Date functions in where cluse..
Is ther any alternate method to use these date functions only in select but not in where clause..

Thanks,
Vijay

Alternate method
select col1,col2,difference from
(Select col1, col2,
fn_date_covert(dat_col1) - fn_date_covert(dat_col2) differnce
From table1, table2
where join (table1, table2))
where difference >=1;