Symptom
Mail always displays message dates and times using U.S. date and time notation. For example: "Wed Feb 26, 2003 5:06:16 Europe/Stockholm".
Solution
You can try this workaround to change Mail to honor the 24-hour clock format instead of the 12-hour clock format.
1. Choose Apple menu > System Preferences.
2. Choose View > International.
3. Click the Language tab (if necessary) and drag English to the top of the list.
4. Choose System Preferences > Quit.
5. Choose Apple menu > Log Out.
6. From the login panel, select or type your user name and password to log in.
7. Choose Apple menu > System Preferences.
8. Choose View > International. Click the Time tab and verify that the Time format is set to the 24-hour clock format
9. Click the Language tab and drag your preferred language to the top of the list.
10. Choose Apple menu > Log out.
11. From the login panel, select or type your user name and password to log in.
12. Mail should now honor the 24-hour clock format using your preferred language.
Note: This only changes the 12-hour clock format to the 24-hour clock format. This will not change the default US Date format used by Mail.