Aggregates in WHERE clause?

I tried to do the following and got the following error message:

select employee,count(distinct tasks)
        from job
        where greatest(max(last_job_date),max(last_position_date)) < 2008-08-28 + integer '1'
        group by employee;

ERROR:  aggregates not allowed in WHERE clause

********** Error **********

ERROR: aggregates not allowed in WHERE clause
SQL state: 42803

Is there away around this? Should i create a function to populate a variable that can be used in it's place...will that even work?