The point of scavenging is to remove old and stale records from a DNS zone. Basically if you do not scavenge the records they will not get removed from the Database, so they will remain in the zone forever. This of course is not an ideal situation especially if you are a large environment, therefore scavenging helps with troubleshooting network problems.

Where do I enable it ?

Scavenging is enabled at the Zone level and also at the Server Level. Scavenging will not run unless enabled at both levels.

Timestamp's

Note however if the zone is not Active Directory Integrated and if scavenging is not enabled at the zone level then the time stamps will not be added to the records. Once scavenging is enabled time stamps are only enabled for NEW records that are then registered. One solution to this is to use the command dnscommand /ageallrecords, however beware !...if you have allot of records which have been manually added and should not be scavenged these will not be excluded from the above command,so consider your options carefully. Take time out to read this article for more information around this important area.

http://technet2.microsoft.com/WindowsServer/en/Library/e312517b-9bbb-4ceb-bef0-117cdcc9ca871033.mspx