My understanding is that the idea is that if you do something that causes the cache mechanism to fail such as renaming a group then you can set the value to the day (or day before) you made the change. This will force the cache to refresh if it precedes the change.

The normal cache validation and 30 day refresh will still happen, but this is a way to force the refresh in the exceptional case that you've made a change that won't be picked up.