Problem to add a delay to a date

Hi,

I try to create a function in charge of adding a delay to a timestamp.
Here are the stange results on Postgres 7.3.1.
working with psql :

test=# select now() + '60' as time;
                time
--------------------------------------
2003-03-24 16:04:53.8680551+01
(1 row)


and when i want to create a function :

test=# create function addtime() returns timestamp as 'select now() +
''60'' ' language 'sql';
ERROR: return type mismatch in function: declared to return timestamp
without time zone, returns timestamp with time zone


Where is the error .

    Thierry