Mac Mail Dates completely wrong

I am having a problem with Mac Mail: the dates are wrong. I am not just talking about a couple of hours. They are completely wrong. For instance today, I have mails which show their date as being 5 September, formatted 05/09/2011 (its 4 September), and when I open them, the actual dates shown range from 1 September to 4 September. For August, ALL my mails show 5 August (05/08/2011). And the same for July: all 5 July. These are the dates shown in the normal column/pane view. When I click on the actual mail, the correct date is shown in the header.

This feels like its a really simple problem. Its a little inconvenient though to have all email dates showing the same for the month. Maybe something to do with date format/time zones...(I am in South Africa, and all the date stuff is set to that).

I have a Macbook Air, OSX Lion.

Any ideas anyone?