Mail puts a January date in email headers - why?

Using OS 10.7.4 the Mail app on my MacBook (late 2008) has started listing the headers of all emails as "January" regardless what month we are in. This occurs only in the body of the email. The day numbers are correct. Strangely enough, the dates given in the column showing the email titles are correct. Can't figure out what happened, but do remember that the date formats started changing (ie number of month instead of letter abbreviation, etc.) and I tried to correct them with the Date and Time custom settings in Sys Prefs. Has anyone seen this before? Fixes?

OK - don't know if this fix will last, or whether the problem will reappear. I went to Date and Time in Sys Prefs, then Open Language then Formats. Under Region I selected Canada, then adjusted my formats, so it ends up reading Custom (just like it did before). Suddenly the proper month started appearing in the body of the email in the header. Weird - hope the problem is gone for good.

I will post this anyhow in case someone out there knows better what is going on with this problem. Thanks ahead of time!