dates in functions

Hello:

I'm trying to define a function that, given a date, returns its month.
The definition is as follows:

CREATE function anyo_hidro (date) returns int AS '
BEGIN
RETURN date_part("month",$1);
END;
' LANGUAGE 'plpgsql';

But when I do:

select anyo_hidro('1-1-1999');

I get the following error:

ERROR: Attribute 'month' not found

Why doesn't date_part work with pl/sql functions?

Thanks

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