Mail.app using Day of the Year for date stamps. Whaaaat?

Here's a new one, for me anyway:

It seems all of my emails in the mail.app have suddenly been stamped with dates that use the day of the year, rather than day of the month, i.e:

37 February, 2011, instead of 6 February, 2011.

This date format appears in both the Viewer Window and Message Headers, and is seen in both sent/received dates, on all saved/new messages (all the way back to a few from 2001) and in both of the mail.app accounts I use: one straight-up gmail, the other a worldshare.net address, also using the Gmail server.

This behavior is not present when reading through webmail, nor is it a problem on my iPod Touch.

Any thoughts?

Thanks.

--iMac 2.16GHz, 3 MB RAM, 250G HD, Snow Leopard 10.6.6