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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