Scheduled action interval doesn't work

Hi, it's me again. I need your help.

 

I have a new requirement. If a Problem record wasn't updated after 15 days, it should send a notification. For that i've created a RuleSet with a Run Scheduled Action, in the Scheduled Action format I set Calc Field to Update Time, Calc Interval to 0 00:05:00 (5 minutes to do a test), Run Rule Set to pbm.notification.closure (for example) and Action after Rule Set to Do nothing. It runs but not at 5 minutes, the notification has sent after 13 seconds.

 

Please help. Any idea? or another method to do that?

Thank you!