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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