Re: to_char incompatibility

small non important note: your function is very expensive

exactly same but faster is:

CREATE OR REPLACE FUNCTION fn_to_date(p_date varchar, p_format varchar)
RETURNS timestamp AS $$
SELECT to_timestamp(replace($1, ' ', ''), replace($2, ' ', ''));
$$ LANGUAGE SQL STRICT IMMUTABLE;

or

CREATE OR REPLACE FUNCTION fn_to_date(p_date varchar, p_format varchar)
RETURNS timestamp AS $$
BEGIN
RETURN to_timestamp(replace(p_date, ' ', ''), replace(p_format, ' ', ''));
END$$ LANGUAGE SQL STRICT IMMUTABLE;

there isn't any reason for using security definer and you forgot IMMUTABLE,

Regards
Pavel Stehule