Problem with aliasing

I wrote a plpgsql function to return the maximum of three dates.

TAL=# select max(req_del_date1, req_del_date2, req_del_date3) as ma1
from invoices;
     ma1
------------
  2003-02-25
  2003-02-25
(2 rows)


However I'm having problems with the following query:

TAL=# select max(req_del_date1, req_del_date2, req_del_date3) as max
from invoices where max <= now();
ERROR:  Attribute "max" not found

Why can't the where part of the query see "max"? I've tried aliasing the
returned value but that did not work either:

TAL=# select max(req_del_date1, req_del_date2, req_del_date3) as ma1
from invoices where ma1 <= now();
ERROR:  Attribute "ma1" not found

What is wrong with my syntax?

Thanks!

Jc