How to convert a text variable into a timestamp in postgreSQL?

I know how to convert a text to timestamp in postgreSQL using

*    SELECT to_timestamp('05 Dec 2000', 'DD Mon YYYY')*

but how can I convert a text variable (inside a function) to timestamp??

In my table (table_ebscb_spa_log04) "time" is a character varying column, in
which I have placed a formated date time (15-11-30 11:59:59.999 PM).
I have tried this function, in order to convert put the date time text into
a variable (it always change) and convert it into timestamp...

*    CREATE OR REPLACE FUNCTION timediff()
      RETURNS trigger AS
    $BODY$
    DECLARE
    timeascharvar character varying;
    timeastistamp timestamp;

    BEGIN
    IF NEW.time_type = 'Lap' THEN
      SELECT t.time FROM table_ebscb_spa_log04 t INTO timeascharvar;
      SELECT to_timestamp('timeascharvar', 'yy-mm-dd HH24:MI:SS.MS') INTO
timeastistamp;
    END IF;
    RETURN timeastistamp;
    END
    $BODY$
      LANGUAGE plpgsql VOLATILE
      COST 100;
    ALTER FUNCTION timediff()
      OWNER TO postgres;*

but whenever I run it in the table, it shows this ERROR message...

<http://postgresql.nabble.com/file/n5855346/screenshot.bmp>

It seems that "to_timestamp" waits for a number to be the year, how can I
get it to recognize the variable as if it were numbers?





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