Incorrect 'last post' dates on forum page

Change the server clock (call it T1) backwards some days and create a forum with a few discussions. Move the clock to current (call it T2) and post a reply to the discussions. Go to the forum page and examine the discussion list. Verify : The "Started by" column shows T1 for all discussions. The "Last post" column shows T2 for all discussions. The tooltip displayed over the "last post" is also T2 (matches the text). Move the clock to tomorrow (call it T3). Go to ONE of the discussions and post another reply. Remember which discussion is. Go to the forum page and examine the discussion list. Verify : The "Started by" column shows T1 for all discussions. The "Last post" column shows T3 for the discussion with the new post. The tooltip displayed over the "last post" is also T3 (matches the text) for that discussion. All the other discussions continue showing "Last post" T2. Like they were in 4) above.