Umm... please dismiss my reply. It doesn't seem to work.
This timer is scheduled to be fired in every 5 secs.

2012-10-16 13:59:37.246 NSOperationTest[1573:303] timer fired..
2012-10-16 13:59:42.247 NSOperationTest[1573:303] timer fired..

2012-10-16 13:59:58.098 NSOperationTest[1573:303] timer fired..
2012-10-16 14:00:02.247 NSOperationTest[1573:303] timer fired..