PostgreSQL - How to create a function to validate a field is DateTime?
Try something like:

to_timestamp('21/12/2008 12:34:23','dd/mm/yyyy HH:MI:SS')


I assume you want to catch any exception and return null instead, so something like:

create or replace function my_to_timestamp(arg text)
returns timestamp language plpgsql
as $$
begin
    begin
        return to_timestamp(arg, 'dd/mm/yyyy HH:MI:SS');
    exception when others then
        return null;
    end;
end $$;


Example:

postgres=# select my_to_timestamp('21/12/2008 12:34:14');
   my_to_timestamp   
---------------------
 2008-12-21 00:34:14
(1 row)


postgres=# select my_to_timestamp('21/12/2008 12:x:14');
 my_to_timestamp 
-----------------

(1 row)


You can find all valid/invalid rows as:

with t(s) as ( values ('21/12/2008 12:34:14')
                    , ('21/12/2008 12:Tx:14')
) 
select s from T
where my_to_timestamp(s) is [not] null;