[MlMt] Small bug in build 2980

[Robert Ricci]

When looking at the message list in threaded mode, the 'Date Received' 
column does a funny thing when expanding/collapsing threads: normally, 
It shows the 'Date Received' column as 'Today', 'Yesterday', etc, but 
when I expand (or collapse) a thread, that column switches to another 
format ('YYYY-MM-DD HH::MM:SS') for a fraction of a second, then 
switches back.

I just updated my Mac to Mountain Lion from Snow Leopard; I'm pretty 
sure it didn't do this under Snow Leopard, but of course I can't easily 
confirm that now. :)