I think this is incorrect in a number of areas.

There is no "period" option on my latest install.

Current:

"Forum Display Period

You can choose whether your forum posting is displayed from a certain date, expires on a certain date or is only displayed over a given period.

Deselect the Disable option(s) to activate a display start and/or end date.

Note that users with Admin access will see messages before their release date and after their expiry date."

Better?

Discussion display period

You can choose whether your the discussion (including replies) is displayed from a specified date and is removed from display before a specified date.

Deselect the Disable option(s) to activate a display start and/or end date.

Note that users with the "View hidden timed posts" capability will be able to view hidden discussions before their release date and after their expiry date.