help with function and quotes

Dear Sirs:
Please I am attempting implement a function that update a 
column to current time + X seconds
So the query involved looks like:
update mytable set mytime=now+'X seconds';
Here X is a integer value,
Now I need implement this on my function, But I have problems 
at implement this at quotes, e.g. my function could looks 
like:
CREATE    FUNCTION addtime(integer) RETURNS INTEGER AS '
DECLARE  additional ALIAS FOR $1;
BEGIN update mytable set mytime=now()+''additional seconds'';
END;
'
return 1;
LANGUAGE 'plpgsql';
I know probably this doesn't  works, justly I have doubts 
about how to implement this since I want the funtion takes 
the variable additional , not the string 'additional', What I 
should to consider? I read the documentation on pqsql 
language aboute using quotes but none case is resemblance to 
my case.
Can someone tell me an orientation about this?
Thank you  in advance.
Ernesto


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