Acknowledged Event logs

Hello.

Am trying to see the event log for the past month, but the system only shows me the events from the same date as of today but in the previous month. How long does NPM keep the acknowledged events in the database? does it discard the events that are more than 30 days old?  even when i do a custom selection on the time period i still cannot get the event logs which are more than 30 days old. Please help.

Thanks