One of the new features in Lion’s Mail is a widescreen-friendly, three-column layout, in which your mailboxes appear on the left, your message list in the middle, and a message preview on the right. But that layout has (at least) one drawback: there’s less room to display information about each message in the middle list. To fit the tighter screen space, Mail shortens the dates it displays for each message. For those received today, it shows only the time the email was received; for those received yesterday, it displays the word Yesterday; and for messages received before that, it shows the date.
While we haven’t yet found a way to display the date and time for messages received yesterday, it’s possible to add the date for messages received today and the time for older messages. The trick is to adjust the settings for displaying times and dates system wide.
Open the Language & Text pane of System Preferences, go to the Formats tab, and click on the Customize button in the Dates section. In the resulting dialog box, set up your preferred format in the Short field, and then select and copy everything in it (Edit > Select All and then Edit > Copy, or Cmd-A followed by Cmd-C). Click Cancel to exit without saving your changes.
Now click the Customize button in the Times section and set up your preferred time format in the Short field there. When it’s the way you want, put the cursor at the end of that field, press the right-arrow key (to be sure nothing is selected), type , (comma) and paste the contents of the Clipboard (Edit > Paste or Cmd-V). The result should look something like 12:00 AM, 5/1/11.
To see the date and time of most messages in Mail, customise the Short date and time formats in System Preferences
Next, select and copy the new Short time format and click OK. Click on the Dates section’s Customize button again. Delete everything in the Short format field, and then paste what you copied from the Times dialog box into it. Click OK. Now, the messages you’ve received today and before yesterday should all display the same time-and-date format.
If you ever decide to go back to the original date and time formats, click the Customize buttons in the Dates and Times sections and, in their respective Short fields, delete the capsules for time and date. Note that these changes will also affect the display of the short date and time elsewhere in OS X – for example, in the Date Modified and Date Created columns in Finder windows.