all of my 'mac mail' inbox mail (except for 'yesterday' and 'today') is dated March 8, 2010 and if i click the date on the top right of the screen (which is the correct date) a window opens with a greyed-out date of March 8. i have OSX 10.6.3
Darn that Preview! Sometimes it doesn't work. Now, if "Mail" had been in your title, I would have known I can't help, other that to suggest a little more specificity in your titles in the future.
It is extremely unlikely that the date in the menu bar is correct, but the full date beneath it is not. Please provide a screen shot of the problem. (screenshot is shift-command-4, then click and drag over the date with the drop down menu)
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