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

Question

this is my table...

In order to autofill 'time_elapse' rows, I would like to get the time difference between the two timestamps rows in a special format: (years/months/days hours:minutes:seconds:miliseconds) like this...

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

Currently Im using this trigger...

CREATE OR REPLACE FUNCTION timediff()
  RETURNS trigger AS
$BODY$
DECLARE
tstemp timestamp;

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 tstemp;
    IF NOT FOUND THEN
      RAISE EXCEPTION USING MESSAGE = 'There is not any previous row...';
    ELSE
      NEW.time_elapse := to_char((NEW.time - tstemp), 'yy-mm-dd HH24:MI:SS.MS');
    END IF;
END IF;
return NEW;
END
$BODY$
  LANGUAGE plpgsql VOLATILE

The trigger works, but there is a kinda unwanted behaviour showed in this img...

The timedifference only counts the days, even if have past a month or a year, it keeps counting only days (483 in the img). I would like that for every 30 days of timedifference add 1 month and subtract 30 days, also for every 12 months of timedifference add 1 year and subtract 12 months, What would be the easiest way to do that?

Thanks Advanced.