Get the difference between two timestamp cells but in a special format in PostgreSQL

This is my table...

<http://postgresql.nabble.com/file/n5855074/screenshot.jpg>

I would like to get the time difference between 'time_type = Lap' AND
'time_type = Start' in order to fill 'time_elapse'. Im almost sure that this
code works...

* CREATE OR REPLACE FUNCTION timediff()
RETURNS trigger AS
$BODY$
DECLARE
t_ix real;

BEGIN
IF NEW.time_type = 'Lap' THEN
SELECT t.time FROM table_ebscb_spa_log04 t WHERE t.fn_name =
NEW.fn_name AND t.time_type = 'Start' ORDER BY t.stmtserial DESC LIMIT 1
INTO t_ix;
IF NOT FOUND THEN
RAISE EXCEPTION USING MESSAGE = 'There is not any previous
row...';
ELSE
NEW.time_elapse := t_ix - NEW.time;
END IF;
END IF;
return NEW;
END
$BODY$
LANGUAGE plpgsql VOLATILE*

But I don't know how to get the time difference between the two timestamps
cells in a special format: (years/months/days
hours:minutes:seconds:miliseconds) like this...

* 0y/0m/0d 00:00:01.001*

Is this possible??
Thanks Advanced.

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