What OS are you using?

OS10 v.10.9.5. The problem persists through OS updates.

What is your locale setting in OSX? TimeZone? Do you have the TZ environment variable set?

Have you right click the folder in question and selected properties and repair?

The System Preferences > Date & Time and Time Zone settings are correct. I don't know anything about the TZ environment variable setting. The dates are incorrect in Thunderbird, not my file system. Which folder is the "folder in question?" Thank you.

Matt said

What is your locale setting in OSX? TimeZone? Do you have the TZ environment variable set? Have you right click the folder in question and selected properties and repair?

Those are direct questions. Please answer then instead of telling me they are correct. Thunderbird uses UTC time as that is the time designated for mails with locale offsets. Your file system uses whatever is set in windows settings. There is no relationship between the two at all. By far the most common reason date in mails are funny are caused by people who do not grasp the fact that their computer thinks in is in the US on Pacific Standard Time (That is the time zone of Microsoft's headquarters are) They set the time and think they have done the job. Note here that the time on mails is generally set by the sender, so lots of the issues are not anything to do with the recipient, but where ALL the mail is wrong by the same amount. The Local timezone information, either through the operating system settings or the TZ environment variable are the cause. Want to know about TZ, Google is your friend.